Presented  to 

THE  BILTMORE  ROOM 
By  Jiqrty   &  I/Vel htj 


TWENTIETH   CENTURY  TEXT-BOOKS 


PLANTS 


A  TEXT-BOOK  OF  BOTANY 


BY 


JOHN   M.   COULTER,   A.M.,   Ph.D. 

HEAD   OF   DEPARTMENT   OF   BOTANY 
UNIVERSITY    OF   CHICAGO 


NEW    YORK 

D.    APPLETON    AND    COMPANY 

1910 


Copyright,  1899 
By  D.   APPLETON  AND  COMPANY 


PLANTS 

A  TEXT-BOOK  OF  BOTANY 


PREFATORY  NOTE 

Although  Plant  Relations  and  Plant  Structures  have 
been  prepared  as  independent  volumes,  chiefly  to  meet  the 
needs  of  those  schools  which  can  give  but  one  half  year  to 
Botany,  they  form  together  a  natural  introduction  to  the 
science.  With  this  in  view,  the  simple  title  Plants  seems 
suitable,  with  the  understanding  that  this  volume  is  an 
introduction  to  the  study  of  plants. 

Either  part  of  this  combined  volume  may  be  used  first, 
according  to  the  views  or  needs  of  the  teacher.  In  many 
cases  it  may  be  wise  not  to  observe  the  order  of  the  book, 
but  to  organize  laboratory  work  as  seems  best,  and  to  assign 
the  appropriate  readings  wherever  they  may  occur  in  the 
volume.  The  author  is  a  stickler  for  independent  teaching, 
and  would  not  presume  to  prescribe  an  order  or  a  method 
for  teachers.  His  purpose  is  simply  to  offer  those  facts  and 
suggestions  which  may  be  helpful  to  them  in  organizing 
and  presenting  their  work.  He  would  urge  that  intelligent 
contact  with  plants  is  the  essential  thing;  that  a  clear 
understanding  of  a  few  large  facts  is  better  than  the  collec- 
tion of  numerous  small  ones ;  and  that  "  getting  through  " 
should  never  sacrifice  the  leisure  needed  for  digestion. 

The  two  parts  of  this  work  are  indexed  separately,  and 
references  to  indexes  are  to  be  made  at  the  end  of  each  part. 

John  M.  Coulter. 

The  University  of  Chicago,  November,  1899. 


TWENTIETH   CENTURY  TEXT- BOOKS 


PLANT  RELATIONS 

A  FIRST  BOOK  OF  BOTANY 


BY 

JOHN  M.  COULTER,  A.M.,  Ph.D 

HEAD    PROFESSOR    OF   BOTANY 
UNIVERSITY  OF  CHICAGO 


THIRD   EDITION  REVISED 


NEW    YORK 

D.    APPLETON    AND    COMPANY 

1910 


Copyright,  1899,  1904, 
By  D.  APPLETON  AND  COMPANY. 


PEEFACE. 

The  methods  of  teaching  botany  in  secondary  schools 
are  very  diverse,  and  in  so  far  as  they  express  the  experience 
of  successful  teachers,  they  are  worthy  of  careful  considera- . 
tion.  As  the  overwhelming  factor  in  successful  teaching 
is  the  teacher,  methods  are  of  secondary  importance,  and 
may  well  vary.  It  is  the  purpose  of  the  present  work  to 
contribute  another  suggestion  as  to  the  method  of  teach- 
ing botany  in  secondary  schools.  The  author  does  not 
intend  to  criticise  other  methods  of  teaching,  for  each 
teacher  has  his  own  best  method,  but  it  may  be  well  to 
state  the  principles  which  underlie  the  preparation  of  this 
work. 

The  botany  is  divided  into  two  parts,  each  representing 
work  for  half  a  year.  The  two  books  are  independent, 
and  opinions  may  differ  as  to  which  should  precede.  The 
first  book,  herewith  presented,  is  dominated  by  Ecology, 
and  also  contains  certain  fundamentals  of  Physiology  that 
are  naturally  suggested.  The  second  book  will  be  domi- 
nated by  Morphology,  but  plant  structure,  function,  and 
classification  will  be  developed  together  in  an  attempt  to 
trace  the  evolution  of  the  plant  kingdom.  In  the  judg- 
ment of  the  author  Ecology  should  precede  Morphology, 
but  this  order  brings  to  Ecology  no  knowledge  of  plant 
structures  and  plant  groups,  which  is  of  course  unfortu- 
nate. The  advantages  which  seem  to  overbalance  this  dis- 
advantage are  as  follows : 

1.  The  study  of  the  most  evident  life-relations  of 
plants  gives  a  proper  conception  of  the  place  of  plants  in 


VI  PREFACE. 

nature,  a  fitting  background  for  subsequent  more  detailed 
studies. 

2.  Such  a  view  of  the  plant  kingdom  is  certainly  of  the 
most  permanent  value  to  those  who  can  give  but  a  half 
year  to  botany,  for  the  large  problems  of  Ecology  are  con- 
stantly presented  in  subsequent  experience,  when  details 
of  structure  would  be  forgotten. 

3.  The  work  in  Ecology  herein  suggested  demands  lit- 
tle or  no  use  of  the  compound  microscope,  an  instrument 
ill  adapted  to  first  contacts  with  nature. 

The  second  book  will  demand  the  use  of  the  compound 
microscope,  and  those  schools  which  possess  such  an  equip- 
ment may  prefer  to  use  that  part  first  or  exclusively. 

In  reference  to  the  use  of  this  part  something  should 
be  said,  although  such  cautions  are  reiterated  in  almost 
every  recent  publication.  A  separate  pamphlet  containing 
"  Suggestions  to  Teachers  "  who  use  this  book  has  been 
prepared,  but  a  few  general  statements  may  be  made  here. 
This  book  is  intended  to  present  a  connected,  readable 
account  of  some  of  the  fundamental  facts  of  botany,  and 
may  serve  to  give  a  certain  amount  of  information.  If  it 
performs  no  other  service  in  the  schools,  however,  its  pur- 
pose will  be  defeated.  It  is  entirely  too  compact  for  any 
such  use,  for  great  subjects,  which  should  involve  a  large 
amount  of  observation,  are  often  merely  suggested.  It  is 
intended  to  serve  as  a  supplement  to  three  far  more  im- 
portant factors  :  (1)  the  teacher,  who  must  amplify  and 
suggest  at  every  point ;  (2)  the  laboratory,  which  must 
bring  the  pupil  face  to  face  with  plants  and  their  struc- 
tures ;  (3)  field-work,  which  must  relate  the  facts  observed 
in  the  laboratory  to  their  actual  place  in  nature,  and  must 
bring  new  facts  to  notice  which  can  be  observed  nowhere 
else.  Taking  the  results  obtained  from  these  three  fac- 
tors, the  book  seeks  to  organize  them,  and  to  suggest 
explanations.  It  seeks  to  do  this  in  two  ways  :  (1)  by 
means  of  the  text,  which  is  intended  to  be  clear  and  un- 


PREFACE.  Vll 

technical,  but  compact ;  (2)  by  means  of  the  illustrations, 
which  must  be  studied  as  carefully  as  the  text,  as  they  are 
only  second  in  importance  to  the  actual  material.  Espe- 
cially is  this  true  in  reference  to  the  landscapes,  many  of 
which  cannot  be  made  a  part  of  experience. 

Thanks  are  due  to  various  members  of  the  botanical 
staff  of  the  University,  who  have  been  of  great  service  in 
offering  suggestions  and  in  preparing  illustrations.  In 
this  first  book  I  would  especially  acknowledge  the  aid  of 
Professor  Charles  R.  Barnes  and  Dr.  Henry  C.  Cowles. 

The  professional  botanist  who  may  critically  examine 
this  first  book  knows  that  Ecology  is  still  a  mass  of  incho- 
ate facts,  concerning  which  we  may  be  said  to  be  making 
preliminary  guesses.  It  seems  to  be  true,  nevertheless, 
that  these  facts  represent  the  things  best  adapted  for  pres- 
entation in  elementary  work.  The  author  has  been  com- 
pelled to  depend  upon  the  writings  of  Warming  and  of 
Kerner  for  this  fundamental  material.  From  the  work  of 
the  latter,  and  from  the  recent  splendid  volume  of  Sehim- 
per,  most  useful  illustrations  have  been  obtained.  The 
number  of  original  illustrations  is  large,  but  those  obtained 
elsewhere  are  properly  credited.  John  M.  Coulter. 

The  University  of  Chicago,  May,  1899. 


PEEFACE   TO   THE   SECOND  EDITION. 

In  this  edition  the  first  eleven  chapters  remain  practi- 
cally as  they  were,  with  the  exception  of  such  corrections 
and  additions  as  could  be  made  upon  the  plates,  and  a  few 
changes  of  illustrations.  The  remaining  chapters,  however, 
dealing  with  plant  societies,  are  essentially  recast  both  in 
text  and  illustrations.  Especially  is  this  true  of  the  meso- 
phyte  and  halophyte  societies.  This  has  been  made  neces- 
sary by  the  recent  rapid  development  of  the  subject,  by  a 
larger  field  experience,  and  by  the  availability  of  more  suit- 
able illustrations.  J.  M.  C. 
The  University  of  Chicago,  May,  1901. 


Vlll  PREFACE. 


PREFACE  TO  THE  THIRD  EDITION. 

During  the  last  three  years  the  science  of  Botany  has 
made  rapid  progress,  both  in  the  addition  of  new  facts 
and  in  changed  points  of  view.  Some  of  this  progress 
affects  Plant  Relations,  and  it  is  recorded  in  this  third 
edition  so  far  as  it  can  be  without  a  complete  rewriting 
of  the  volume.  Changes  will  be  found,  therefore,  in  state- 
ments of  fact,  in  points  of  view,  in  terminology,  in  illus- 
trations, and  also  in  the  addition  of  new  material. 

John  M.  Coulter. 
The  University  of  Chicago,  July,  1904. 


CONTENTS. 

CHATTER  PAGE 

I. — Introduction    .                1 

II. — Foliage  leaves  :    The  light-relation          ...  6 
III. — Foliage  leaves  :    Function,  structure,  and  protection     28 

IV. — Shoots 53 

V.— Roots 89 

VI. — Reproductive  organs 109 

VII. — Flowers  and  insects      .......  123 

VIII. — An  individual  plant  in  all  of  its  relations      .        .  138 

IX. — The  struggle  for  existence 142 

X. — The  nutrition  of  plants 149 

XI. — Plant  associations:  Ecological  factors       ,        ,        ,  162 

XII. — Hydrophyte  associations 170 

XIII. — Xerophyte  associations    ..,.»,,  193 

XIV. — Mesophyte  associations    ...,,.,  233 

Index •       •       .  259 


BOTANY 

PART    I.— PLANT    RELATIONS 


CHAPTER    I. 

INTRODUCTION. 

1.  General  relations. — Plants  form  the  natural  covering 
of  the  earth's  surface.  So  generally  is  this  true  that  a  land 
surface  without  plants  seems  remarkable.  Not  only  do 
plants  cover  the  land,  but  they  abound  in  waters  as  well, 
both  fresh  and  salt  waters.  They  are  wonderfully  varied  in 
size,  ranging  from  huge  trees  to  forms  so  minute  that  the 
microscope  must  be  used  to  discover  them.  They  are  also 
exceedingly  variable  in  form,  as  may  be  seen  by  comparing 
trees,  lilies,  ferns,  mosses,  mushrooms,  lichens,  and  the 
green  thready  growths  {alga)  found  in  water. 

2.  Plant  associations.— One  of  the  most  noticeable  facts 
in  reference  to  plants  is  that  they  do  not  form  a  monot- 
onous covering  for  the  earth's  surface,  but  that  there  are 
forests  in  one  place,  thickets  in  another,  meadows  in 
another,  swamp  growths  in  another,  etc.  In  this  way  the 
general  appearance  of  vegetation  is  exceedingly  varied, 
and  each  appearance  tells  of  certain  conditions  of  living. 
These  groups  of  plants  living  together  in  similar  conditions, 
as  trees  and  other  plants  in  a  forest,  or  grasses  and  other 
plants  in  a  meadow,  are  known  as  plant  associations.    These 


2  PLANT  RELATIONS. 

associations  are  as  numerous  as  are  the  conditions  of  living, 
and  it  may  be  said  that  each  association  has  its  own  special 
regulations,  which  admit  certain  plants  and  exclude  others. 
The  study  of  plant  associations,  to  determine  their  conditions 
of  living,  is  one  of  the  chief  purposes  of  botanical  field  work. 

3.  Plants  as  living  things. — Before  engaging  in  a  study 
of  associations,  however,  one  must  discover  in  a  general  way 
how  the  individual  plant  lives,  for  the  plant  covering  of  the 
earth's  surface  is  a  living  one,  and  plants  must  always  be 
thought  of  as  living  and  at  work.  They  are  as  much  alive 
as  are  animals,  and  so  far  as  mere  living  is  concerned  they 
live  in  much  the  same  way.  Nor  must  it  be  supposed  that 
animals  move  and  plants  do  not,  for  while  more  animals  than 
plants  have  the  power  of  moving  from  place  to  place,  some 
plants  have  this  power,  and  those  that  do  not  can  move  cer- 
tain parts.  The  more  we  know  of  living  things  the  more  is 
it  evident  that  life  processes  are  alike  in  them  all,  whether 
plants  or  animals.  In  fact,  there  are  some  living  things 
about  which  we  are  uncertain  whether  to  regard  them  as 
plants  or  animals. 

4.  The  plant  body. — Every  plant  has  a  body,  which  may 
be  alike  throughout  or  may  be  made  up  of  a  number  of 
different  parts.  When  the  green  thready  plants  (algai),  so 
common  in  fresh  water,  are  examined,  the  body  looks  like 
a  simple  thread,  without  any  special  parts  ;  but  the  body  of 
a  lily  is  made  up  of  such  dissimilar  parts  as  root,  stem, 
leaf,  and  flower  (see  Figs.  75,  144,  155,  174).  The  plant 
without  these  special  parts  is  said  to  be  simple,  the  plant 
with  them  is  called  complex.  The  simple  plant  lives  in 
the  same  way  and  does  the  same  kind  of  work,  so  far  as 
living  is  concerned,  as  does  the  complex  plant.  The  differ- 
ence is  that  in  the  case  of  the  simple  plant  its  whole  body 
does  every  kind  of  work  ;  while  in  the  complex  plant 
different  kinds  of  work  are  done  by  different  regions  of  the 
body,  and  these  regions  come  to  look  unlike  when  differ- 
ent shapes  are  better  suited  to  different  work,  as  in  the 


INTRODUCTION.  3 

case  of  a  leaf  and  a  root,  two  regions  of  the  body  doing 
different  kinds  of  work. 

5.  Plant  organs. — These  regions  of  the  plant  body  thus 
set  apart  for  special  purposes  are  called  organs.  The  sim- 
plest of  plants,  therefore,  do  not  have  distinct  organs, 
while  the  complex  plants  may  have  several  kinds  of  organs. 
All  plants  are  not  either  very  simple  or  very  complex,  but 
beginning  with  the  simplest  plants  one  may  pass  to  others 
not  quite  so  simple,  then  to  others  more  complex,  and  so 
on  gradually  until  the  most  complex  forms  are  reached. 
This  process  of  becoming  more  and  more  complex  is  known 
as  differentiation,  which  simply  means  the  setting  apart  of 
different  regions  of  the  body  to  do  different  kinds  of  work. 
The  advantage  of  this  to  the  plant  becomes  plain  by  using 
the  common  illustration  of  the  difference  between  a  tribe 
of  savages  and  a  civilized  community.  The  savages  all  do 
the  same  things,  and  each  savage  does  everything.  In  the 
civilized  community  some  of  the  members  are  farmers, 
others  bakers,  others  tailors,  others  butchers,  etc.  This  is 
what  is  known  as  "  division  of  labor,"  and  one  great  advan- 
tage it  has  is  that  every  kind  of  work  is  better  done.  Dif- 
ferentiation of  organs  in  a  plant  means  to  the  plant  just 
what  division  of  labor  means  to  the  community  ;  it  results 
in  more  work,  and  better  work,  and  new  kinds  of  work. 
The  very  simple  plant  resembles  the  savage  tribe,  the  com- 
plex plant  resembles  the  civilized  community.  It  must  be 
understood,  however,  that  in  the  case  of  plants  the  differ- 
entiation referred  to  is  one  of  organs  and  not  of  individuals. 

6.  Plant  functions. — Whether  plants  have  many  organs, 
or  few  organs,  or  no  organs,  it  should  be  remembered  that 
they  are  all  at  work,  and  are  all  doing  the  same  essential 
things.  Although  many  different  kinds  of  work  are  being 
carried  on  by  plants,  they  may  all  be  put  under  two  heads, 
nutrition  and  reproduction.  Every  plant,  whether  simple 
or  complex,  must  care  for  two  things  :  (1)  its  own  support 
(nutrition),  and  (2)  the  production  of   other  plants  like 

2 


4  PLANT   EELATIONS. 

itself  (reproduction).  To  the  great  work  of  nutrition  many 
kinds  of  work  contribute,  and  the  same  is  true  of  repro- 
duction. Nutrition  and  reproduction,  however,  are  the 
two  primary  kinds  of  work,  and  it  is  interesting  to  note 
that  the  first  advance  in  the  differentiation  of  a  simple 
plant  body  is  to  separate  the  nutritive  and  reproductive 
regions.  In  the  complex  plants  there  are  nutritive  organs 
and  reproductive  organs  ;  by  which  is  meant  that  there  are 
distinct  organs  which  specially  contribute  to  the  work  of 
nutrition,  and  others  which  are  specially  concerned  with 
the  work  of  reproduction.  The  different  kinds  of  work  are 
conveniently  spoken  of  as  functions,  each  organ  having  one 
or  more  functions. 

7.  Life-relations. — In  its  nutritive  and  reproductive  work 
the  plant  is  very  dependent  upon  its  surroundings.  It 
must  receive  material  from  the  outside  and  get  rid  of  waste 
material ;  and  it  must  leave  its  offspring  in  as  favorable 
conditions  for  living  as  possible.  As  a  consequence,  every 
organ  holds  a  definite  relation  to  something  outside  of  it- 
self, known  as  its  life-relation.  For  example,  green  leaves 
are  definitely  related  to  light,  many  roots  are  related  to 
soil,  certain  plants  are  related  to  abundant  water,  some 
plants  are  related  to  other  plants  or  animals  (living  as 
parasites),  etc.  A  plant  with  several  organs,  therefore, 
may  hold  a  great  variety  of  life-relations,  and  it  is  quite  a 
complex  problem  for  such  a  plant  to  adjust  all  of  its  parts 
properly  to  their  necessary  relations.  The  study  of  the 
life-relations  of  plants  is  a  division  of  Botany  known  as 
Ecology,  and  presents  to  us  many  of  the  most  important 
problems  of  plant  life. 

It  must  not  be  supposed  that  any  plant  or  organ  holds 
a  perfectly  simple  life-relation,  for  it  is  affected  by  a  great 
variety  of  things.  A  root,  for  instance,  is  affected  by  light, 
gravity,  moisture,  soil  material,  contact,  etc.  Every  or- 
gan, therefore,  must  adjust  itself  to  a  very  complex  set  of 
life-relations,  and  a  plant  with  several  organs  has  so  many 


INTRODUCTION.  5 

delicate  adjustments  to  care  for  that  it  is  really  impossi- 
ble, as  yet,  for  us  to  explain  why  all  of  its  parts  are  placed 
just  as  they  are.  In  the  beginning  of  the  study  of  plants, 
only  some  of  the  most  prominent  functions  and  life-rela- 
tions can  be  considered.  In  order  to  do  this,  it  seems  bet- 
ter to  begin  with  single  organs,  and  afterwards  these  can 
be  put  together  in  the  construction  of  the  whole  plant. 


CHAPTER  II. 

FOLIAGE  LEAVES:    THE  LIGHT-RELATION. 

8.  Definition. — A  foliage  leaf  is  the  ordinary  green  leaf, 
and  is  a  very  important  organ  in  connection  with  the  work 
of  nutrition.  It  must  not  be  thought  that  the  work  done  by 
such  a  leaf  cannot  be  done  by  green  plants  which  have  no 
leaves,  as  the  algae,  for  example.  A  leaf  is  simply  an  or- 
gan set  apart  to  do  such  work  better.  In  studying  the 
work  of  a  leaf,  therefore,  we  have  certain  kinds  of  work 
set  apart  more  distinctly  than  if  they  were  confused  with 
other  kinds.  For  this  reason  the  leaf  is  selected  as  an  in- 
troduction to  some  of  the  important  work  carried  on  by 
plants,  but  it  must  not  be  forgotten  that  a  plant  does  not 
need  leaves  to  do  this  Avork  ;  they  simply  enable  it  to  work 
more  effectively. 

9.  Position. — It  is  easily  observed  that  foliage  leaves 
grow  only  upon  stems,  and  that  the  stems  which  bear  them 
always  expose  them  to  light ;  that  is,  such  leaves  are  aerial 
rather  than  subterranean  (see  Figs.  1,  75,  174).  Many 
stems  grow  underground,  and  such  stems  either  bear  no 
foliage  leaves,  or  are  so  placed  that  the  foliage  leaves  are 
sent  above  the  surface,  as  in  most  ferns  and  many  plants  of 
the  early  spring  (see  Figs.  45,  46,  144). 

10.  Color. — Another  fact  to  be  observed  is  that  foliage 
leaves  have  a  characteristic  green  color,  a  color  so  universal 
that  it  has  come  to  be  associated  with  plants,  and  espe- 
cially with  leaves.  It  is  also  evident  that  this  green  color 
holds  some  necessary  relation  to  light,  for  the  leaves  of 
plants  grown  in  the  dark,  as  potatoes  sprouting  in  a  cellar, 


FOLIAGE   LEAVES:    THE   LIGHT-RELATION.  7 

do  not  develop  this  color.  Even  when  leaves  have  devel- 
oped the  green  color  they  lose  it  if  deprived  of  light,  as  is 
shown  by  the  process  of  blanching  celery,  and  by  the  effect 
on  the  color  of  grass  if  a  board  has  lain  upon  it  for 
some  time.  It  seems  plain,  therefore,  that  the  green  color 
found  in  working  foliage  leaves  depends  upon  light  for  its 
existence. 

We  conclude  that  at  least  one  of  the  essential  life-rela- 
tions of  a  foliage  leaf  is  what  may  be  called  the  light-rela- 
tion. This  seems  to  explain  satisfactorily  why  such  leaves 
are  not  developed  in  a  subterranean  position,  as  are  many 
stems  and  most  roots,  and  why  plants  which  produce  them 
do  not  grow  in  the  dark,  as  in  caverns.  The  same  green, 
and  hence  the  same  light-relation,  is  observed  in  other 
parts  of  the  plant  as  well,  and  in  plants  without  leaves,  the 
only  difference  being  that  leaves  display  it  most  conspicu- 
ously. Another  indication  that  the  green  color  is  con- 
nected with  light  may  be  obtained  from  the  fact  that  it  is 
found  only  in  the  surface  region  of  plants.  If  one  cuts 
across  a  living  twig  or  into  a  cactus  body,  the  green  color 
will  be  seen  only  in  the  outer  part  of  the  section.  The  con- 
clusion is  that  the  leaf  is  a  special  organ  for  the  light-re- 
lation. Plants  sometimes  grow  in  such  situations  that  it 
would  be  unsafe  for  them  to  display  leaves,  or  at  least  large 
leaves.  In  such  a  case  the  work  of  the  leaves  can  be  thrown 
upon  the  stem.  A  notable  illustration  of  this  is  the  cactus 
plant,  which  produces  no  foliage  leaves,  but  whose  stem  dis- 
plays the  leaf  color. 

11.  An  expanded  organ. — Another  general  fact  in  refer- 
ence to  the  foliage  leaf  is  that  in  most  cases  it  is  an  expanded 
organ.  This  means  that  it  has  a  great  amount  of  surface 
exposed  in  comparison  with  its  mass.  As  this  form  is  of 
such  common  occurrence  it  is  safe  to  conclude  that  it  is  in 
some  way  related  to  the  work  of  the  leaf,  and  that  whatever 
work  the  leaf  does  demands  an  exposure  of  surface  rather 
than  thickness  of  body.     It  is  but  another  step  to  say  that 


8 


PLANT   RELATIONS. 


the  amount  of  work  an  active  leaf  can  do  will  depend  in 
part  upon  the  amount  of  surface  it  exposes. 


THE   LIGHT-RELATION. 


12.  The  general  relation. — The  ordinary  position  of  the 
foliage  leaf  is  more  or  less  horizontal.  This  enables  it  to 
receive  the  direct  rays  of  light  upon  its  upper  surface.     In 

this  way  more  rays  of 
light  strike  the  leaf  sur- 
face than  if  it  stood  ob- 
liquely or  on  edge.  It  is 
often  said  that  leaf  blades 
are  so  directed  that  the 
flat  surface  is  at  right 
angles  to  the  incident 
rays  of  light.  Wbile  this 
may  be  true  of  horizon- 
tal leaves  in  a  general 
way,  the  observation  of 
almost  any  plant  will 
show  that  it  is  a  very 
general  statement,  to 
which  there  are  numerous 
exceptions  (see  Fig.  1). 
Leaves  must  be  arranged 
to  receive  as  much  light 
as  possible  to  help  in 
their  work,  but  too  much 
light  will  destroy  the 
green  substance  {chloro- 
phyll), which  is  essential 
to  the  work.  The  adjust- 
ment to  light,  therefore, 
is  a  delicate  one,  for 
there  must  be  just  enough 


Fig.  1.  The  leaves  of  this  plant  (Ficus)  are 
in  general  horizontal,  but  it  will  be  seen 
that  the  lower  ones  are  directed  down- 
ward, and  that  the  leaves  become  more 
horizontal  as  the  stem  is  ascended.  It 
will  also  be  seen  that  the  leaves  are  so 
broad  that  there  are  few  vertical  rows. 


FOLIAGE   LEAVES:    THE    LIGHT-RELATION.  9 

and  not  too  much.  The  danger  from  too  much  light  is 
not  the  same  in  the  case  of  all  leaves,  even  on  the  same 
plant,  for  some  are  more  shaded  than  others.  Leaves  also 
have  a  way  of  protecting  themselves  from  too  intense  light 
hy  their  structure,  rather  than  by  a  change  in  their  posi- 
tion. It  is  evident,  therefore,  that  the  exact  position  which 
any  particular  leaf  holds  in  relation  to  light  depends  upon 
many  circumstances,  and  cannot  be  covered  by  a  general 
rule,  except  that  it  seeks  to  get  all  the  light  it  can  without 
danger. 

13.  Fixed  position. — Leaves  differ  very  much  in  the  power 
of  adjusting  their  position  to  the  direction  of  the  light. 


Fig.  2.    The  day  and  night  positions  of  the  leaves  of  a  member  (Amicia)  of  the  pea 
family. — After  Strasburger. 

Most  leaves  when  fully  grown  are  in  a  fixed  position  and 
cannot  change  it,  however  unfavorable  it  may  prove  to  be, 
except  as  they  are  blown  about.  Such  leaves  are  said  to 
ha,\e  fixed  light  positions.  This  position  is  determined  by 
the  light  conditions  that  prevailed  while  the  leaf  was  grow- 
ing and  able  to  adjust  itself.  If  these  conditions  continue, 
the  resulting  fixed  position  represents  the  best  one  that  can 
be  secured  under  the  circumstances.  The  leaf  may  not 
receive  the  rays  of  light  directly  throughout  the  whole 
period  of  daylight,  but  its  fixed  position  is  such  that  it 
probably  receives  more  light  than  it  would  in  any  other 
position  that  it  could  secure. 


10 


PLANT  RELATIONS. 


14.  Motile  leaves. — There  are  leaves,  however,  which 
have  no  fixed  light  position,  but  are  so  constructed  that 
they  can  shift  their  position  as  the  direction  of  the  light 
changes.     Such  leaves  are  not  in  the  same  position  in  the 

afternoon  as  in  the 
forenoon,  and  their 
night  position  may  be 
very  different  from 
either  (see  Figs.  2,  3a, 
35,  1).  Some  of  the 
common  house  plants 
show  this  power.  In 
the  case  of  the  com- 
mon Oxalis  the  night 

Fig.  3a.    The  day  position  of  the  leaves  of  redbud         \ 

(Cercis).— After  Arthur.  is  remarkably  different 


from  the  position  in  light. 
If  such  a  plant. is  exposed 
to  the  light  in  a  window  and 
the  positions  of  the  leaves 
noted,  and  then  turned 
half  way  around,  so  as  to 
bring  the  other  side  to  the 
light,  the  leaves  may  be 
observed  to  adjust  them- 
selves gradually  to  the 
changed  light-relations. 

15.    Compass    plants. — A 
striking    illustration    of    a 

special  light  position  is  found  in  the  so-called  "  compass 
plants."  The  best  known  of  these  plants  is  the  rosin- weed 
of  the  prairie  region.  Growing  in  situations  exposed  to 
intense  light,  the  leaves  are  turned  edgewise,  the  flat  faces 
being  turned  away  from  the  intense  rays  of  midday,  and 
directed  towards  the  rays  of  less  intensity  ;  that  is,  those  of 


Fig.  Zb.    The  night  position  of  the  leaves 
of  redbud  (Cercis). — After  Arthur. 


FOLIAGE   LEAVES;    THE   LIGHT-RELATION.  11 


Fig.  4.  Two  sensitive  plants,  showing  the  motile  leaves.  The  plant  to  the  left  lias  its 
leaves  and  numerous  leaflets  expanded  ;  the  one  to  the  right  shows  the  leaflets 
folded  together  and  the  leaves  drooping.— After  Kerner, 

the  morning  and  evening  (see  Fig.  170).  As  a  result,  the 
plane  of  the  leaf  lies  in  a  general  north  and  south  direc- 
tion. It  is  a  significant  fact  that  when  the  plant  grows  in 
shaded  places  the  leaves  do  not  assume  any  such  position. 
It  seems  evident,  therefore,  that  the  position  has  something 
to  do  with  avoiding  the  danger  of  too  intense  light.     It 


12 


PLANT   RELATIONS. 


Fig.  5.  The  common  prickly  lettuce  (Lactuca 
Scariola),  showing  the  leaves  standing  edge- 
wise, and  in  a  general  north  and  south  plane. 
—After  Arthur  and  MacDougal. 


must  not  be  supposed 
that  there  is  any  ac- 
curacy in  the  north  or 
south  direction,  as  the 
edgewise  position 
seems  to  be  the  signifi- 
cant one.  In  the  ros- 
in-weed probably  the 
north  and  south  direc- 
tion is  the  prevailing 
one ;  but  in  the  prickly 
lettuce,  a  very  common 
weed  of  waste  grounds, 
and  one  of  the  most 
striking  of  the  compass 
plants,  the  edgewise 
position  is  frequently 
assumed  without  any 
special  reference  to  the 
north  or  south  direc- 
tion of  the  apex  (see 
Fig.  5). 

16.  Heliotropism. — 
The  property  of  leaves 
and  of  other  organs 
of  responding  to  light 
is  known  as  heliotro- 
pism, and  it  is  one 
of  the  most  important 
of  those  external  influ- 
ences to  which  plant 
organs  respond  (see 
Figs.  6,  43). 

It  should  be  under- 
stood clearly  that  this 
is  but  a  slight  glimpse 


FOLIAGE   LEAVES:    THE   LIGHT-KELAT10.N.  13 


Pig.  6.    These  plants  are  growing  near  a  window.    It  will  be  noticed  that  the  stems 
bend  strongly  towards  the  light,  and  that  the  leaves  face  the  light. 

of  the  most  obvious  relations  of  foliage  leaves  to  light,  and 
that  the  important  part  which  heliotropism  plays,  not  only 
in  connection  with  foliage  leaves,  but  also  in  connection 
with  other  plant  organs,  is  one  of  the  most  important  and 
extensive  subjects  of  plant  physiology. 


RELATION    OF    LEAVES   TO    ONE    ANOTHER. 

A.    On  erect  stems. 

In  view  of  what  has  been  said,  it  would  seem  that  the 
position  of  foliage  leaves  on  the  stem,  and  their  relation  to 
one  another,  must  be  determined  to  some  extent  by  the 
necessity  of  a  favorable  light-relation.  It  is  apparent  that 
the  conditions  of  the  problem  are  not  the  same  for  an  erect 
as  for  a  horizontal  stem. 

17.  Relation  of  breadth  to  number  of  vertical  rows. — 
Upon  an  erect  stem  it  is  observed  that  the  leaves  are  usu- 


14 


PLANT   RELATIONS. 


ally  arranged  in  a  definite  number  of  vertical  rows.  It  is 
to  the  advantage  of  the  plant  for  these  leaves  to  shade  one 
another  as  little  as  possible.  Therefore,  the  narrower  the 
leaves,  the  more  numerous  may  be  the  vertical  rows  (see 

Figs.  7.  8)  ;  and 
the  broader  the 
leaves  the  fewer 
the  vertical  rows 
(see  Fig.  1).  A_ 
relation  exists, 
therefore,  be- 
tween the  breadth 
of  leaves  and  the 
number  of  verti- 
cal rows,  and  the 
meaning  of  this 
becomes  plain 
when  the  light-re- 
lation is  consid- 
ered. 

18.  Relation  of 
length,  to  the  dis- 
tance between 
leaves  of  the  same 
row. — The  leaves 
in  a  vertical  row 
may  be  close  together  or  far  apart.  If  they  should  be  close 
together  and  at  the  same  time  long,  it  is  evident  that  they 
will  shade  each  other  considerably,  as  the  light  cannot  well 
strike  in  between  them  and  reach  the  surface  of  the  lower 
leaf.  Therefore,  the  closer  together  the  leaves  of  a  verti- 
cal row,  the  shorter  are  the  leaves  ;  and  the  farther  apart 
the  leaves  of  a  row,  the  longer  may  they  be.  Short  leaves 
permit  the  light  to  strike  between  them  even  if  they  are 
close  together  on  the  stem ;  and  long  leaves  permit  the 
same  thing  only  when  they  are  far  apart  on  the  stem.     A 


Fig.  7. 


An  Easter  lily,  showing  narrow  leaves  and 
numerous  vertical  rows. 


FOLIAGE    LEAVES  :    THE    LIGHT-RELATION 


15 


relation  is  to  be  observed,  therefore,  between  the  length 
of  leaves  and  their  distance  apart  in  the  same  vertical  row. 
The  same  kind  of  relation  can  be  observed  in  reference 
to  the  breadth  of  leaves,  for  if  leaves  are  not  only  short  but 
narrow  they  can  stand  very  close  together.  It  is  thus  seen 
that  the  length  and  breadth  of  leaves,  the  number  of  ver- 
tical rows  on  the  stem,  and  the  distance  between  the  leaves 


Fig.  8.    A  dragon-tree,  showing  narrow  leaves  extending  in  all  directions,  and  numer- 
ous vertical  rows. 


of  any  row,  all  have  to  do  with  the  light-relation  and  are 
answers  to  the  problem  of  shading. 

19.  Elongation  of  the  lower  petioles. — There  is  still 
another  common  arrangement  by  which  an  effective  light- 
relation  is  secured  by  leaves  which  are  broad  and  placed 
close  together  on  the  stem.  In  such  a  case  the  stalks 
(petioles)  of  the  lower  leaves  become  longer  than  those 
above  and  thus  thrust  their  blades  beyond  the  shadow  (see 
Fig.   9).      It  may  be  noticed  that  it  is  very  common    to 


16  PLANT  RELATIONS. 

find  the  lowest  leaves  of  a  plant  the  largest  and  with  the 
longest  petioles,  even  when  the  leaves  are  not  very  close 
together  on  the  stem. 

It  must  not  be  supposed  that  by  any  of  these  devices 
shading  is  absolutely  avoided.  This  is  often  impossible  and 
sometimes   undesirable.     It  simply   means  that   by  these 


Fig.  9.    A  plant  (Saintpaulia)  with  the  lower  petioles  elongated,  thrusting  the  blades 
beyond  the  shadow  of  the  upper  leaves.    A  loose  rosette. 


arrangements  the  most  favorable  light-relation  is  sought  by 
avoiding  too  great  shading. 

20.  Direction  of  leaves. — Not  only  is  the  position  on  the 
stem  to  be  observed,  but  the  direction  of  leaves  may  result 
in  a  favorable  relation  to  light.  It  is  a  very  common  thing 
to  find  a  plant  with  a  cluster  of  comparatively  large  leaves 
at  or  near  the  base,  where  they  are  in  no  danger  of  shading 
other  leaves,  and  with  the  stem  leaves  gradually  becoming 


FOLIAGE   LEAVES  :    THE   LIGHT-RELATION. 


17 


smaller  and  less  horizontal  toward  the  apex  of  the  stem 
(see  Figs.  10,  13).  The  common  shepherd's  purse  and  the 
mullein  may  be  taken  as  illustrations.  By  this  arrange- 
ment all  the  leaves  are  very 
completely  exposed  to  the 
light. 

21.  The  rosette  habit. — 
The  habit  of  producing  a 
cluster  or  rosette  of  leaves 
at  the  base  of  the  stem  is 
called  the  rosette  habit. 
Often  this  rosette  of  leaves 
at  the  base,  frequently  lying 
flat  on  the  ground  or  on  the 
rocks,  includes  the  only  fo- 
liage leaves  the  plant  pro- 
duces. It  is  evident  that  a 
rosette,  in  which  the  leaves 
must  overlap  one  another 
more  or  less,  is  not  a  very 
favorable  light  arrange- 
ment, and  therefore  it  must 
be  that  something  is  being 
provided  for  besides  the 
light-relation  (see  Figs.  11, 
12,  13).  What  this  is  will 
appear   later,    but   even   in 

this  comparatively  unfavorable  light  arrangement,  there  is 
evident  adjustment  to  secure  the  most  light  possible  under 
the  circumstances.  The  lowest  leaves  of  the  rosette  are 
the  longest,  and  the  upper  (or  inner)  ones  become  gradu- 
ally shorter,  so  that  all  the  leaves  have  at  least  a  part 
of  the  surface  exposed  to  light.  The  overlapped  base  of 
such  leaves  is  not  expanded  as  much  as  the  exposed  apex, 
and  hence  they  are  mostly  narrowed  at  the  base  and  broad 
at  the  apex.     This  narrowing  at   the   base   is   sometimes 


Fig.  10.  A  plant  (Echeveria)  with  fleshy 
leaves,  showing  large  horizontal  ones 
at  base,  and  others  becoming  smaller 
and  more  directed  upward  as  the 
stem  is  ascended. 


18 


PLANT   KELATIONS. 


carried  so  far  that  most  of  the  part  which  is  covered  is 
but  a  stem  (petiole)  for  the  upper  part  (blade)  which  is 
exposed. 

In  many  plants  which  do  not  form  close  rosettes  a  gen- 


Fig.  11.  A  group  of  live-for-evers.  fllnstratine  the  rosette  habit  and  the  light-relation. 
In  the  rosettes  it  will  be  observed  how  the  leaves  are  fitted  together  and  diminish 
in  size  inwards,  so  that  excessive  shading  is  avoided.  The  individual  leaves  also 
become  narrower  where  they  overlap,  and  are  broadest  where  they  are  exposed  to 
light.    In  the  background  is  a  plant  showing  leaves  in  very  definite  vertical  rows. 


3ral  rosette  arrangement  of  the  leaves  may  be  observed  by 
looking  down  upon  them  from  above  (see  Fig.  9),  as  in  some 
of  the  early  buttercups  which  are  so  low  that  the  large 
leaves  would  seriously  shade  one  another,  except  that  the 
lower  leaves  have  longer  petioles  than  the  upper,  and  so 
reach  beyond  the  shadow. 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION. 


19 


Fig.  12.  Two  clumps  of  rosettes  of  the  house  leek  (Semperritmrn),  the  one  to  the 
right  showing  the  compact  winter  condition,  the  one  to  the  left  with  rosettes  more 
open  after  being  kept  indoors  for  several  days. 


22.  Branched  leaves. — Another  notable  feature  of  foliage 
leaves,  which  has  something  to  do  with  the  light-relation, 
is  that  on  some  plants  the  blade  does  not  consist  of  one 
piece,  but  is  lobed  or  even  broken  up  into  separate  pieces. 
When  the  divisions  are  distinct  they  are  called  leaflets,  and 
every  gradation  in  leaves  can  be  found,  from  distinct  leaf- 
lets to  lobed  leaves,  toothed  leaves,  and  finally  those  whose 
margins  are  not  indented  at  all  {entire).  This  difference 
in  leaves  probably  has 
more  important  rea- 
sons than  the  light- 
relation,  but  its  sig- 
nificance may  be  ob- 
served in  this  connec- 
tion. In  those  plants 
whose  leaves  are  un- 
divided, the  leaves 
generally  either  di- 
minish in  size  toward 
the  top  of  the  stem, 
or  the  lower  ones  de- 
velop longer  petioles. 
In  this  case  the  gen- 
eral outline  of  the 
3 


Fig.  13.  The  leaves  of  a  bellflower  {Campanula), 
showing  the  rosette  arrangement.  The  lower 
petioles  are  successively  longer,  carrying  their 
blades  beyond  the  shadow  of  the  blades  above. 
—After  Kebneb. 


Fig.  14.  A  group  of  leaves,  showing  how  branched  leaves  overtop  each  other  without 
dangerous  shading.  It  will  be  seen  that  the  larger  blades  or  less-branched  leaves 
are  towards  the  bottom  of  the  group. 


FOLIAGE   LEAVES:    THE   LIGHT- RELATION. 


21 


plant  is  conical,  a  form  very  common  in  herbs  with  entire 
or  nearly  entire  leaves.  In  plants  whose  leaf  blades  are 
broken  up  into  leaflets  {compound  or  branched  leaves), 
however,  no  such  diminution  in  size  toward  the  top  of  the 
stem  is  necessary  (see  Fig.  17),  though  it  may  frequently 


Fig.  15.    A  plant  showing  much-branched  leaves,  which  occur  in  great  profusion  with- 
out cutting  off  the  light  from  one  another. 


occur.  When  a  broad  blade  is  broken  up  into  leaflets 
the  danger  of  shading  is  very  much  less,  as  the  light  can 
strike  through  between  the  upper  leaflets  and  reach  the 
leaflets  below.  On  the  lower  leaves  there  will  be  splotches 
of  light  and  shadow,  but  they  will  shift  throughout  the 
day,  so  that  probably  a  large  part  of  the  leaf  will  receive 
light  at  some  time   during  the  day  (see  Fig.   14).     The 


22 


PLANT   RELATIONS. 


general  outline  of  such  a  plant,  therefore,  is  usually  not 
conical,  as  in  the  other  case,  but  cylindrical  (see  Figs.  4, 
15,  16,  22,  45,  83,  90,  155,  162,  174  for  branched  leaves). 

Many  other  factors  enter  into  the  light-relation  of  foli- 
age leaves  upon  erect  stems,  but  those  given  may  suggest 


Fig.  16.    A  cycad,  showing  much-branched  leaves  ana  palm-like  habit. 

observation  in  this  direction,  and  serve  to  show  that  the 
arrangement  of  leaves  in  reference  to  light  depends  upon 
many  things,  and  is  by  no  means  a  fixed  and  indifferent 
thing.  The  study  of  any  growing  plant  in  reference  to  this 
one  relation  presents  a  multitude  of  problems  to  those  who 
know  how  to  observe. 


B.    On  horizontal  stems. 

23.  Examples  of  horizontal  stems,  that  is,  stems  exposed 
on  one  side  to  the  direct  light,  will  be  found  in  the  case  of 
many  branches  of  trees,  stems  prostrate  on  the  ground,  and 


FOLIAGE   LEAVES:    THE    LIGHT-RELATION. 


23 


stems  against  a  support,  as  the  ivies.  It  is  only  necessary 
to  notice  how  the  leaves  are  adjusted  to  light  on  an  erect 
stem,  and  then  to  bend  the 
stem  into  a  horizontal  posi- 
tion or  against  a  support,  to 
realize  how  unfavorable  the 
same  arrangement  would 
be,  and  how  many  new  ad- 
justments must  be  made. 
The  leaf  blades  must  all  be 
brought  to  the  light  side  of 
the  stem,  so  far  as  possible, 
and  those  that  belong  to 
the  lower  side  of  the  stem 
must  be  fitted  into  the 
spaces  left  by  the  leaves 
which  belong  to  the  upper 
side.  This  may  be  brought 
about  by  the  twisting  of. 
the  stem,  the  twisting  of 
the  petioles,  the  bending  of 
the  blade  on  the  petiole, 
the  lengthening  of  petioles, 
or  in  some  other  way. 
Every  horizontal  stem  has 
its  own  special  problems  of 
leaf  adjustment  which  may 
be  observed  (see  Figs.  18, 
50). 

Sometimes  there  is  not 
space  enough  for  the  full 
development  of  every  blade, 
and  smaller  ones  are  fitted 
into  the  spaces  left  by  the  larger  ones  (see  Fig.  21).  This 
sometimes  results  in  what  are  called  unequally  paired  leaves, 
where  opposite  leaves  develop  one  large  blade  and  one  small 


Fig.  17.  A  chrysanthemum,  showing 
lobed  leaves,  the  rising  of  the  petioles 
to  adjust  the  blades  to  light,  and  the 
general  cylindrical  habit. 


24 


PLANT   RELATIONS. 


one.  Perhaps  the  most  complete  fitting  together  of  leaves 
is  found  in  certain  ivies,  where  a  regular  layer  of  angular 
interlocking  leaves  is  formed,  the  leaves  fitting  together  like 


Fig.  18.    A  plant  (Pellionia)  with  drooping  stems,  showing  how  the  leaves  are  all 
brought  to  the  lighted  side  and  fitted  together. 

the  pieces  of  a  mosaic.  In  fact  such  an  arrangement  is 
known  as  the  mosaic  arrangement,  and  involves  such  an 
amount  of  twisting,  displacement,  elongation  of  petioles, 


26 


PLANT  RELATIONS. 


Fig.  20.  A  spray  of  maple,  showing  the  adjustment  of  the  leaves  in  size  and  position 
of  blades  and  length  of  petioles  to  secure  exposure  to  light  on  a  horizontal  stem.— 
After  Kerner. 


etc.,  as  to  give  ample  evidence  of  the  effort  put  forth  by 
plants  to  secure  a  favorable  light-relation  for  their  foliage 


Fig.  21.  Two  plants  showing  adjustment  of  leaves  on  a  horizontal  stem.  The  plant 
to  the  left  is  nightshade,  in  which  small  blades  are  fitted  into  spaces  left  by  the 
large  ones.  The  plant  to  the  right  is  Selaginella,  in  which  small  leaves  are  dis- 
tributed along  the  Bides  of  the  stem,  and  others  arc  displayed  along  the  upper  sur- 
face.— After  Kerner. 


FOLIAGE   LEAVES:    THE  LIGHT-RELATION. 


27 


leaves  (see  Figs.  19,  22).  In  the  case  of  ordinary  shade  trees 
every  direction  of  branch  may  be  found,  and  the  resulting 
adjustment  of  leaves  noted  (see  Fig.  20). 

Looking  up  into  a  tree  in  full  foliage,  it  will  be  noticed 
that  the  horizontal  branches  are   comparatively  bare  be- 


Pig.  22.    A  mosaic  of  fern  (Adiantum)  leaflets. 


neath,  while  the  leaf  blades  have  been  carried  to  the  upper 
side  and  have  assumed  a  mosaic  arrangement. 

Sprays  of  maidenhair  fern  (see  Fig.  22)  show  a  remark- 
able amount  of  adjustment  of  the  leaflets  to  the  light  side. 
Another  group  of  fern-plants,  known  as  club-mosses,  luis 
horizontal  stems  clothed  with  numerous  very  small  leaves. 
These  leaves  may  be  seen  taking  advantage  of  all  the  space 
on  the  lighted  side  (see  Fig.  21). 


CHAPTER  III. 

FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    AND    PROTEC- 
TION. 

A.     Functions  of  foliage  leaves. 

24.  Functions  in  general. — We  have  observed  that  foliage 
leaves  are  light-related  organs,  and  that  this  relation  is  an 
important  one  is  evident  from  the  various  kinds  of  adjust- 
ment used  to  secure  it.  We  infer,  therefore,  that  for  some 
important  function  of  these  leaves  light  is  necessary.  It 
would  be  hasty  to  suppose  that  light  is  necessary  for  every 
kind  of  work  done  by  a  foliage  leaf,  for  some  forms  of  work 
might  be  carried  on  by  the  leaf  that  light  neither  helps  nor 
hinders.  Foliage  leaves  are  not  confined  to  one  function, 
but  are  concerned  in  a  variety  of  processes,  all  of  which 
have  to  do  with  the  great  work  of  nutrition.  Among  the 
variety  of  functions  which  belong  to  foliage  leaves  some  of 
the  most  important  may  be  selected  for  mention.  It  will 
be  possible  to  do  little  more  than  indicate  these  functions 
until  the  plant  with  all  its  organs  is  considered,  but  some 
evidence  can  be  obtained  that  various  processes  are  taking 
place  in  the  foliage  leaf. 

25.  Photosynthesis. — The  most  important  function  of  the 
foliage  leaf  may  be  detected  by  a  simple  experiment.  If 
an  actively  growing  water  plant  submerged  in  water  in  a 
glass  vessel  be  exposed  to  bright  light,  bubbles  may  be  seen 
coming  from  the  leaf  surfaces  and  rising  through  the  water 
(see  Eig.  23).  The  water  is  merely  a  device  by  which  the 
bubbles  of  gas  may  be  seen.     If  the  plant  is  very  active  the 


FOLIAGE   LEAVES:    FUNCTION,  STEDCTUEE,  ETC. 


29 


bubbles  are  numerous.  That  this  activity  holds  a  definite 
relation  to  light  may  be  proved  by  shading  the  vessel  con- 
taining the  plant.  When  the  light  is  diminished  the  bub- 
bles diminish  in  number,  and  when  sufficiently  darkened 


Fig.  23.    An  experiment  to  illustrate  the  giving  off  of  oxygen  in  the  process  of  photo- 
synthesis. 

the  bubbles  will  cease  entirely.  If  now  the  vessel  be  again 
illuminated,  the  bubbles  will  reappear,  and  the  rapidity 
with  which  the  bubbles  are  formed  will  indicate  in  a  rough 
way  the  activity  of  the  process.  That  this  gas  being  given 
off  is  mainly   oxygen  may  be  proved  by  collecting  the 


30  PLANT  RELATIONS. 

bubbles  (by  inverting  over  the  plants  a  large  funnel  and 
leading  them  into  a  test  tube),  and  testing  it  in  the  usual 
way. 

Some  very  important  things  are  learned  by  this  experi- 
ment. It  is  evident  that  some  process  is  going  on  within 
the  leaves  which  needs  light  and  which  results  in  giving  off 
oxygen.  It  is  further  evident  that  as  oxygen  is  eliminated, 
the  process  indicated  is  dealing  with  substances  which 
contain  more  oxygen  than  is  needed.  The  amount  of 
oxygen  given  off  may  be  taken  as  the  measure  of  the  work. 
The  more  oxygen,  the  more  work ;  and,  as  we  have  observed, 
the  more  light,  the  more  oxygen ;  and  no  light,  no  oxygen. 
Therefore,  light  must  be  essential  to  the  work  of  which  the 
elimination  of  oxygen  is  an  external  indication.  That  this 
process,  whatever  it  may  be,  is  so  essentially  related  to 
light,  suggests  the  idea  that  it  is  the  special  process  which 
demands  that  the  leaf  shall  be  a  light-related  organ.  If  so, 
it  is  a  dominating  kind  of  work,  as  it  chiefly  determines 
the  life-relations  of  foliage  leaves. 

The  process  thus  indicated  is  known  as  photosynthesis, 
and  the  name  suggests  that  it  has  to  do  with  the  arrange- 
ment of  material  with  the  help  of  light.  It  is  really  a  pro- 
cess of  food  manufacture,  by  which  raw  materials  are  made 
into  plant  food.  This  process  is  an  exceedingly  important 
one,  for  upon  it  depend  the  lives  of  all  plants  and  animals. 
The  foliage  leaves  may  be  considered,  therefore,  as  special 
organs  of  photosynthesis.  They  are  special  organs,  not  ex- 
clusive organs,  for  any  green  tissue,  whether  on  stem  or  fruit 
or  any  part  of  the  plant  body,  may  do  the  same  work.  It 
is  at  once  apparent,  also,  that  during  the  night  the  process 
of  photosynthesis  is  not  going  on,  and  therefore  during  the 
night  oxygen  is  not  being  given  off. 

Another  part  of  this  process  is  not  so  easily  observed,  but 
is  so  closely  related  to  the  elimination  of  oxygen  that  it 
must  be  mentioned.  Carbon  dioxide  occurs  in  the  air  to 
which  the  foliage  leaves  are  exposed.     It  is  given  off  from 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       31 

our  lungs  in  breathing,  and  also  comes  off  from  burning 
wood  or  coal.  It  is  a  common  waste  product,  being  a  com- 
bination of  carbon  and  oxygen  so  intimate  that  the  two 
elements  are  separated  from  one  another  with  great  dif- 
ficulty. During  the  process  of  photosynthesis  it  has  been 
discovered  that  carbon  dioxide  is  being  absorbed  from  the 
air  by  the  leaves.  As  this  gas  is  absorbed  chiefly  by  green 
parts  and  in  the  light,  in  just  the  conditions  in  which  oxy- 
gen is  being  given  off,  it  is  natural  to  connect  the  two,  and 
to  infer  that  the  process  of  photosynthesis  involves  not  only 
the  green  color  and  the  light,  but  also  the  absorption  of 
carbon  dioxide  and  the  elimination  of  oxygen. 

When  we  observe  that  carbon  dioxide  is  a  combination 
of  carbon  and  oxygen,  it  seems  reasonable  to  suppose  that 
the  carbon  and  oxygen  are  separated  from  one  another  in 
the  plant,  and  that  the  carbon  is  retained  and  the  oxygen 
given  back  to  the  air.  The  process  of  photosynthesis  may 
be  partially  defined,  therefore,  as  the  breaking  up  of  carbon 
dioxide  by  the  green  parts  of  the  plants  in  the  presence  of 
light,  the  retention  of  the  carbon,  and  the  elimination  of 
the  oxygen.  The  carbon  retained  is  combined  into  real 
plant  food,  in  a  way  to  be  described  later.  We  may  con- 
sider photosynthesis  as  the  most  important  function  of  the 
foliage  leaf,  of  which  the  absorption  of  carbon  dioxide  and 
the  evolution  of  oxygen  are  external  indications  ;  and  that 
light  and  chlorophyll  are  in  some  way  essentially  connected 
with  it. 

26.  Transpiration. — One  of  the  easiest  things  to  observe 
in  connection  with  a  working  leaf  is  the  fact  that  it  gives 
off  moisture.  A  simple  experiment  may  demonstrate  this. 
If  a  glass  vessel  (bell  jar)  be  inverted  over  a  small  active 
plant  the  moisture  is  seen  to  condense  on  the  glass,  and 
even  to  trickle  down  the  sides.  A  still  more  convenient  way 
to  demonstrate  this  is  to  select  a  single  vigorous  leaf  with 
a  good  petiole  ;  pass  the  petiole  through  a  perforated  card- 
board resting  upon  a  tumbler  containing  water,  and  invert 


32  PLANT   RELATIONS. 

a  second  tumbler  over  the  blade  of  the  leaf,  which  projects 
above  the  cardboard  (see  Pig.  24).  It  will  be  observed  that 
moisture  given  off  from  the  surface  of  the  working  leaf  is 
condensed  on  the  inner  surface  of  the  inverted  tumbler. 
The  cardboard  is  to  shut  off  evaporation  from  the  water 
in  the  lower  tumbler. 

When  the  amount  of  water  given  off  by  a  single  leaf  is 
noted,  some  vague  idea  may  be  formed  as  to  the  amount  of 
moisture  given  off  by  a  great  mass  of  vegetation,  such  as  a 
meadow  or  a  forest.  It  is  evident  that  green  plants  at 
work  are  contributing  a  very  large  amount  of  moisture  to 
the  air  in  the  form  of  water  vapor,  moisture  which  has 
been  absorbed  by  some  region  of  the  plant.  The  foli- 
age leaf,  therefore,  may  be  regarded  as  an  organ  of 
transpiration,  not  that  the  leaves  alone  are  engaged  in 
transpiration,  for  many  parts  of  the  plant  do  the  same 
thing,  but  because  the  foliage  leaves  are  the  chief  seat  of 
transpiration. 

In  case  the  leaves  are  submerged,  as  is  true  of  many 
plants,  it  is  evident  that  transpiration  is  practically  checked, 
for  the  leaves  are  already  bathed  with  water,  and  under  such 
circumstances  water  vapor  is  not  given  off.  It  is  evident 
that  under  such  circumstances  leaf  work  must  be  carried 
on  without  transpiration.  In  some  cases,  as  in  certain 
grasses,  fuchsias,  etc.,  drops  of  water  are  extruded  at  the 
apex  of  the  leaf,  or  at  the  tips  of  the  teeth.  This  process 
is  called  guttation,  and  by  means  of  it  a  good  deal  of 
water  passes  from  the  leaf.  It  is  specially  used  by  shade 
plants,  which  live  in  conditions  which  do  not  favor  tran- 
spiration. 

27.  Respiration. — Another  kind  of  work  also  may  be 
detected  in  the  foliage  leaf,  but  not  so  easily  described. 
In  fact  it  escaped  the  general  attention  of  botanists  much 
longer  than  did  photosynthesis  and  transpiration.  It  is 
work  that  goes  on  so  long  as  the  leaf  is  alive,  never  ceasing 
day  or  night.    The  external  indication  of  it  is  the  absorption 


Pig.  24.    Experiment  illustrating  transpiration. 


34  PLANT  RELATIONS. 

of  oxygen  and  the  giving  out  of  carbon  dioxide.  It  will  be 
noted  at  once  that  this  is.  exactly  the  reverse  of  what  takes 
place  in  photosynthesis.  During  the  day,  therefore,  carbon 
dioxide  and  oxygen  are  both  being  absorbed  and  evolved. 
It  will  also  be  noted  that  the  taking  in  of  oxygen  and  the 
giving  out  of  carbon  dioxide  is  just  the  sort  of  exchange 
which  takes  place  in  our  own  respiration.  In  fact  this  pro- 
cess is  also  called  respiration  in  plants.  It  does  not  depend 
upon  light,  for  it  goes  on  in  the  dark.  It  does  not  depend 
upon  chlorophyll,  for  it  goes  on  in  plants  and  parts  of  plants 
which  are  not  green.  It  is  not  peculiar  to  leaves,  but  goes 
on  in  every  living  part  of  the  plant.  A  process  which  goes 
on  without  interruption  in  all  living  plants  and  animals 
must  be  very  closely  related  to  their  living.  We  conclude, 
therefore,  that  while  photosynthesis  is  peculiar  to  green 
plants,  and  only  takes  place  in  them  when  light  is  present, 
respiration  is  necessary  to  all  plants  in  all  conditions,  and 
that  when  it  ceases  life  must  soon  cease.  The  fact  is, 
respiration  supplies  the  energy  which  enables  the  living 
substance  to  work. 

Once  it  was  thought  that  plants  differ  from  animals 
in  the  fact  that  plants  absorb  carbon  dioxide  and  give  off 
oxygen,  while  animals  absorb  oxygen  and  give  off  carbon 
dioxide.  It  is  seen  now  that  there  is  no  such  difference, 
but  that  respiration  (absorption  of  oxygen  and  evolution  of 
carbon  dioxide)  is  common  to  both  plants  and  animals. 
The  difference  is  that  green  plants  have  the  added  work  of 
photosynthesis. 

We  must  also  think  of  the  foliage  leaf,  therefore,  as  a 
respiring  organ,  because  very  much  of  such  work  is  done 
by  it,  but  it  must  be  remembered  that  respiration  is  going 
on  in  every  living  part  of  the  plant. 

This  by  no  means  completes  the  list  of  functions  that 
might  be  made  out  for  foliage  leaves,  but  it  serves  to  indi- 
cate both  their  peculiar  work  (photosynthesis)  and  the  fact 
that  they  are  doing  other  kinds  of  work  as  well. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       35 

B.  Structure  of  foliage  leaves. 

28.  Gross  structure. — It  is  evident  that  the  essential  part 
of  a  foliage  leaf  is  its  expanded  portion  or  blade.    Often  the 

leaf  is  all  blade  (see  Figs.  7, 
8, 18)  ;  frequently  there  is  a 
longer  or  shorter  leaf-stalk 
{petiole)  which  helps  to  put 


Fig.  25.  Two  types  of  leaf  venation.  The  figure  to  the  left  is  a  leaf  of  Solomon's 
seal  (Polygonalum),  and  shows  the  principal  veins  parallel,  the  very  minute  cross 
veinlets  being  invisible  to  the  naked  eye,  being  a  monocotyl  type.  The  figure  to 
the  Tight  is  a  leaf  of  a  willow,  and  shows  netted  veins,  the  main  central  vein  (mid- 
rib) sending  out  a  series  of  parallel  branches,  which  are  connected  with  one  another 
by  a  network  of  veinlets,  being  a  dicotyl  type. — After  Ettingshatjsen. 

the  blade  into  better  light-relation  (see  Figs.  1,  9,  17,  20, 
2G);  and  sometimes  there  are  little  leaf -like  appendages  {stip- 
ules) on  the  petiole  where  it  joins  the  stem,  whose  func- 
tion is  not   always   clear.     Upon  examining  the  blade  it 

is  seen  to  consist  of  a  green  substance   through  which  a 
4 


36 


PLANT   RELATIONS. 


framework  of  veins  is  variously  arranged.  The  large  veins 
which  enter  the  blade  send  off  smaller  branches,  and  these 
send  off  still  smaller  ones,  until  the  smallest  veinlets  are 

invisible,  and  the 
framework  is  a 
close  network  of 
branching  veins. 
This  is  plainly 
shown  by  a  "skel- 
eton "  leaf,  one 
which  has  been  so 
treated  that  all 
the  green  sub- 
stance has  disap- 
peared, and  only 
the  network  of 
veins  remains.  It 
will  be  noticed 
that  in  some 
leaves  the  veins 
and  veinlets  are 
very  prominent, 
in  others  only 
the  main  veins 
are  prominent, 
while  in  some  it 
is  hard  to  detect 
any  veins  (see 
Figs.  25,  26). 

29.  Significance 
of  leaf  veins. — It 
is  clear  that  the 
framework  of  veins  is  doing  at  least  two  things  for  the 
blade:  (1)  it  mechanically  supports  the  spread  out  green  sub- 
stance ;  and  (2)  it  conducts  material  to  and  from  the  green 
substance.      So  complete  is  the  network  of  veins  that  this 


Fig.  26.  A  leaf  of  hawthorn,  showing  a  short  petiole,  and 
a  broad  toothed  blade  with  a  conspicuous  network  of 
veins.  Note  the  relation  between  the  veins  and  the 
teeth. — After  Strasburger. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       37 

support  and  conduction  are  very  perfect  (see  Fig.  27).  It 
is  also  clear  that  the  green  substance  thus  supported  and 
supplied  with  material  is  the  important  part  of  the  leaf,  the 
part  that  demands  the  light-relation.  Study  the  various 
plans  of  the  vein  systems  in  Figs.  3,  9,  13,  18,  19,  20,  21, 
25,  26,  51,  70,  76,  82,  83,  92,  161. 


Fie.  27.   A  plant  (Fittonia)  whose  leaves  show  a  network  of  veins,  and  also  an  adjust- 
ment to  one  another  to  form  a  mosaic. 

30.  Epidermis. — If  a  thick  leaf  be  taken,  such  as  that 
of  a  hyacinth,  it  will  be  found  possible  to  peel  off  from 
its  surface  a  delicate  transparent  skin  (epidermis).  This 
epidermis  completely  covers  the  leaf,  and  generally  shows 
no  green  color.  It  is  a  protective  covering,  but  at  the  same 
time  it  must  not  completely  shut  off  the  green  substance 
beneath  from  the  outside.  It  is  found,  therefore,  that 
three  important  parts  of  an  ordinary  foliage  leaf  are  :  (1) 


38 


PLANT   RELATIONS. 


Fig.  28.  Cells  of  the  epidermis 
of  Maranta,  showing  the 
interlocking  walls,  and  a 
stoma  (a)  with  its  two  guard- 
cells. 


a  network  of  veins ;  (2)  a  green  substance  (mesophyll)  in 

the  meshes  of  the  network  ;  and  (3)  over  all  an  epidermis. 
31.  Stomata. — If  a  compound  microscope  is  used,  some 

very  important  additional  facts  may  be  discovered.     The 

thin,  transparent  epidermis  is 
found  to  be  made  up  of  a  layer  of 
cells  which  fit  closely  together, 
sometimes  dovetailing  with  each 
other.  Curious  openings  in  the 
epidermis  will  also  be  discovered, 
sometimes  in  very  great  numbers. 
Guarding  each  opening  are  two 
crescent-shaped  cells,  known  as 
guard-cells,  and  between  •  them  a 
slit-like  opening  leads  through  the 
epidermis.  The  whole  apparatus 
is  known  as  a  stoma  (plural 
stomata),     which     really     means 

"  mouth/'  of  which  the  guard-cells  might  be  called  the 

lips  (see  Figs.  28,  29).     Sometimes  stomata  are  found'only 

on  the  under  side  of  the  leaf,  sometimes 

only  on  the  upper  side,  and  sometimes  on 

both  sides. 

One  important  fact  about  stomata  is  that 

the  guard-cells  can  change  their  shape,  and 

so  regulate  the  size  of  the  opening.    It  is  not 

certain  just  why  the  guard-cells  change  their 

shape  and  just  what  stomata  do  for  leaves. 

They  are  often  called  "  breathing  pores,"  but 

a  better  name  would  be  air  pores.     Stomata 

are  not  peculiar  to  the  epidermis  of  foliage 

leaves,  for  they  are  found  in  the  epidermis 

of  any  green  part,  as  stems,  young  fruit, 

etc.     It  is  evident,  therefore,  that  they  hold 

an  important  relation  to  green  tissue  which 

is  covered  by  epidermis.    Also,  if  we  examine 


Fig.  29.  A  single 
stoma  from  the 
epidermis  of  a 
lily  leaf,  show- 
i  n  g  the  two 
guard-cells  full 
of  chlorophyll, 
and  the  small 
slit-like  opening 
between. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.        39 

foliage  leaves  and  other  green  parts  of  plants  which  live 
submerged  in  water,  we  find  that  the  epidermis  contains 
no  stomata.  Therefore,  stomata  hold  a  definite  relation 
to  green  parts  covered  by  epidermis  only  when  this  epider- 
mis is  exposed  to  the  air. 

It  would  seem  that  the  stomata  supply  open  passage- 
ways for  material  from  the  green  tissue  through  the  epider- 
mis to  the  air,  or  from  the  air  to  the  green  tissue,  or  both. 
It  will  be  remembered,  however,  that  quite  a  number  of 
substances  are  taken  into  the  leaf  and  given  out  from  it, 
so  that  it  is  hard  to  determine  whether  the  stomata  are 
specially  for  any  one  of  these  movements.  For  instance, 
the  leaf  gives  out  moisture  in  transpiration,  oxygen  in 
photosynthesis,  and  carbon  dioxide  in  respiration  ;  while  it 
takes  in  carbon  dioxide  in  photosynthesis,  and  oxygen  in 
respiration.  It  is  thought  that  stomata  specially  favor 
transpiration,  and  that  they  also  much  facilitate  the  en- 
trance of  carbon  dioxide. 

32.  Mesophyll. — If  a  cross-section  be  made  of  an  ordi- 
nary foliage  leaf,  such  as  that  of  a  lily,  the  three  leaf 
regions  can  be  seen  in  their  proper  relation  to  each  other. 
Bounding  the  section  above  and  below  is  the  layer  of  trans- 
parent epidermal  cells,  pierced  here  and  there  by  stomata, 
marked  by  their  peculiar  guard-cells.  Between  the  epi- 
dermal layers  is  the  green  tissue,  known  as  the  mesophyll, 
made  up  of  cells  which  contain  numerous  small  green 
bodies  which  give  color  to  the  whole  leaf,  and  are  known  as 
chlorophyll  bodies  or  chloroplasts. 

The  mesophyll  cells  are  usually  arranged  differently  in 
the  upper  and  lower  regions  of  the  leaf.  In  the  upper 
region  the  cells  are  elongated  and  stand  upright,  present- 
ing their  narrow  ends  to  the  upper  leaf  surface,  forming 
the  palisade  tissue.  In  the  lower  region  the  cells  are  irreg- 
ular, and  so  loosely  arranged  as  to  leave  passageways  for  air 
between,  forming  the  spongy  tissue.  The  air  spaces  among 
the  cells  communicate  with  one  another,  so  that  a  system  of 


40 


PLANT   RELATIONS. 


air  chambers  extends  throughout  the  spongy  mesophyll. 
It  is  into  this  system  of  air  chambers  that  the  stomata 
open,  and  so  they  are  put  into  direct  communication  with 
the  mesophyll  or  working  cells.  The  peculiar  arrangement 
of  the  upper  mesophyll,  to  form  the  palisade  tissue,  has  to 
do  with  the  fact  that  that  surface  of  the  leaf  is  exposed  to 
the  direct  rays  of  light.  This  light,  so  necessary  to  the 
mesophyll,  is  also  dangerous  for  at  least  two  reasons.     If 


Fig.  30.  A  section  through  the  leaf  of  lily,  showing  upper  epidermis  (ue),  lower  epi- 
dermis (fe)  with  its  stomata  (st),  mesophyll  (dotted  cells)  composed  of  the  palisade 
region  ^p)  and  the  spongy  region  (sp)  with  airspaces  among  the  cells,  and  two 
veins  (v)  cut  across. 


the  light  is  too  intense  it  may  destroy  the  chlorophyll,  and 
the  heated  air  may  dry  out  the  cells.  The  narrow  ends  of 
the  cells  present  less  exposure,  and  the  depth  of  the  cells 
permits  greater  freedom  of  movement  to  the  chloroplasts. 

33.  Veins. — In  the  cross-section  of  the  leaf  there  will 
also  be  seen  here  and  there,  embedded  in  the  mesophyll, 
the  cut  ends  of  the  veinlets,  made  up  partly  of  thick- 
walled  cells,  which  hold  the  leaf  in  shape  and  conduct 
material  to  and  from  the  mesophyll  (see  Fig.  30). 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTUKE,    ETC.       41 


C.    Leaf  protection. 

34.  Need  of  protection. — Such  an  important  organ  as 
the  leaf,  with  its  delicate  active  cells  well  displayed,  is  ex- 
posed to  numerous  dangers.  Chief  among  these  dangers 
are  intense  light,  drought,  and  cold.  All  leaves  are  not 
exposed  to  these  dangers.  For  example,  plants  which  grow 
in  the  shade  are  not  in  danger  from  intense  light ;  many 

water  plants  are  not  in  danger 
from  drought ;  and  plants  of 
the  tropical  lowlands  are  in  no 


US.  us. 

Fig.  31.  Sections  through  leaves  of  the  same  plant,  showing  the  effect  of  exposure  to 
light  upon  the  structure  of  the  mesophyll.  In  both  cases  os  indicates  upper  surface, 
and  us  under  surface.  In  the  section  at  the  left  the  growing  leaf  was  exposed  to 
direct  and  intense  sunlight,  and,  as  a  consequence,  all  of  the  mesophyll  cells  have 
assumed  the  protected  or  palisade  position.  In  the  section  at  the  right  the  leaf  was 
grown  in  the  shade,  and  none  of  the  mesophyll  cells  have  organized  in  palisade 
fashion. — After  Stahl. 

danger  from  cold.  The  danger  from  all  these  sources  is  be- 
cause of  the  large  surface  with  no  great  thickness  of  body, 
and  the  protection  against  all  of  them  is  practically  the 
same.  Most  of  the  forms  of  protection  can  be  reduced 
to  two  general  plans:  (1)  the  development  of  protective 
structures  between  the  endangered  mesophyll  and  the  air ; 
(2)  the  diminution  of  the  exposed  surface. 

35.  Protective  structures. — The  palisade  arrangement  of 
mesophyll  may  be  regarded  as  an  adaptation  for  protection, 


42 


PLANT  KELATIONS. 


but  it  usually  occurs,  and  does  not  necessarily  imply  ex- 
treme conditions  of  any  kind.  However,  palisade  tissue  of 
unusually  narrow  and  elongated  cells,  or  forming  two  01 


Fia.  32.    Section  through  a  portion  of  the  leaf  of  the  yew  (Taxus),  showing  cuticle 
(c),  epidermis  (e),  and  the  upper  portion  of  the  palisade  cells  (p). 


three  layers,  indicates  exposure  to  intense  light  or  drouth, 
and  is  very  characteristic  of  alpine  and  desert  plants.  The 
accompanying  illustration  (Fig.  31)  shows  in  a  striking 
way  the  effect  of  light  intensity  upon  the  structure  of  the 
mesophyll,  by  contrasting  leaves  of  the  same  plant  exposed 
to  the  extreme  conditions  of  light  and  shade. 

The  most  usual  structural  adaptations,  however,  are 
connected  with  the  epidermis.  The  outer  walls  of  the  epi- 
dermal cells  may  become  thickened,  sometimes  excessively 

so ;  the  other  epidermal 
walls  may  also  become 
more  or  less  thickened  ; 
or  even  what  seems  to 
be  more  than  one  epi- 
dermal layer  is  found 
protecting  the  meso- 
phyll.    If    the    outer 

Fig.  33.    Section  through  a  portion  of  the  leaf    walls    of    the    epidermal 

of  carnation,  showing  the  heavy  cuticle  («/)  11  nn-n  +  ln-n  a      +/\ 

.  ,    ,        .  ,,        -.,  .,  .Cello        U  U II  I  1  11  U  c        to 

formed  by  the  outer  walls  of  the  epidermal 

cells  (ep).   Through  the  cuticle  a  passageway  thicken,    the     outer     re- 

leads  to  the  stoma,  whose  two  guard-cells  are  p-jon    of    the     thick    Wall 

seen  lying  between  the  two  epidermal  cells  ° 

shown  in  the  figure.     Below  the  epidermal  lOSeS      its      Structure 

cells  some  of  the  palisade  cells  (pal)  are  shown  o„J     forms     the     CUticl& 

containing  chloroplasts,  and  below  the  stoma  .     ,       . 

is  seen  the  air  chamber  into  which  it  opens.  W  n  1 C  n    IS     One     Ol     tne 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,   ETC.       43 


best  protective  substances  (see  Fig. 
32).  Sometimes  this  cuticle  be- 
comes so  thick  that  the  passage- 
ways through  it  leading  down  to 
the  stomata  become  regular  canals 
(see  Fig.  33). 

Another  very  common  protective 
structure  upon  leaves  is  to  be  found 
in  the  great  variety  of  hairs  de- 
veloped by  the  epidermis.  These 
may  form  but  a  slightly  downy 
covering,  or  the  leaf  may  be  cov- 
ered by  a  woolly  or  felt-like  mass 
so  that  the  epidermis  is  entirely 
concealed.  The  common  mullein 
is  a  good  illustration  of  a  felt- 
covered  leaf  (see  Fig.  36).  In  cold 
or  dry  regions  the  hairy  covering 
of  leaves  is  very  noticeable,  often 
giving  Jiem  a  brilliant  silky  white  or  bronze  look  (see 
Figs.  34,  35).  Sometimes,  instead  of  a  hair-like  cover- 
ing, the  epidermis  develops  scales  of  various  patterns, 
often  overlapping,  and  forming  an  excellent  protection 
(see  Fig.  37).  In  all  these  cases  it  should  be  remembered 
that  these  hairs  and  scales  may  serve  other  purposes  also, 
and  may  even  be  of  no  use  whatever  to  the  plant. 
36.  Diminution 


of  exposed  surface. — 

It  will  be  impossible 
to  give  more  than  a 
feAv  illustrations  of 
this  large  subject. 
In  very  dry  regions 
it  has  always  been 
noticed  that  the 
leaves  are  small  and 


Fig.  34.  A  hair  from  the  leaf 
of  Potentilla.  It  is  seen 
to  grow  out  from  the  epi- 
dermis. 


Fig.  35.  A  section  through  the  leaf  of  bush  clover 
!  Lespedeza),  showing  upper  and  lower  epidermis, 
palisade  cells,  and  cells  of  the  spongy  region. 
The  lower  epidermis  produces  numerous  hairs 
which  bend  sharply  and  lie  along  the  leaf  surface 
(appressed),  forming  a  close  covering. 


44 


PLANT   RELATIONS. 


Fig.  36.    A  branching  hair  from  the  leaf  of  common  mullein.    The  illustration  shows 
the  form,  but  not  the  many-celled  structure  of  the  hair. 

comparatively  thick,  although  they  may  be  very  numerous 
(see  Figs.  4,  172).      In  this  way  each  leaf  exposes  a  small 

surface  to  the  dry- 
ing air  and  intense 
sunlight.  In  our 
southwestern  dry 
regions  the  cactus 
abounds,  plants 
which  have  reduced 
their  leaves  so  much 
that  they  are  no 
longer  used  for 
chlorophyll  work, 
and  are  not  usually 
recognized  as  leaves. 
In  their  stead  the 
globular  or  cylin- 
drical or  flattened 
stems  are  green  and 

Fig.  37.    A  scale  from  the  leaf  of  Shepherdia.  TheBe 

scales  overlap  and  form  a  complete  covering.  CIO    leal    WOl'K    (rigs. 


Pig.  39.  A  group  of  cactus  forms  (slender  cylindrical,  columnar, 
and  globular),  all  of  them  spiny  and  without  leaves  ;  an  agave  in 
front ;  clusters  of  yucca  flowers  in  the  background. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.        47 

38,  39,  40,  185,  18G,  187,  188).  In  the  same  regions  the 
agaves  and  yuccas  retain  their  leaves,  but  they  become  so 
thick  that  they  serve  as  water  reservoirs  (see  Figs.  38,  39, 


Fig.  40.    A  globular  cactus,  showing  the  ribbed  stem,  the  strong  spines,  and  the  entire 
absence  of  leaves. 

189).  In  all  these  cases  this  reduced  surface  is  supple- 
mented by  palisade  tissue,  very  thick  epidermal  Avails,  and 
an  abundant  cuticle. 

37.  Rosette  arrangement. — The  rosette  arrangement  of 
leaves  is  a  very  common  method  of  protection  used  by 


48 


PLANT   RELATIONS. 


small  plants  growing  in  exposed  situations,  as  bare  rocks 
and  sandy  ground.  The  cluster  of  leaves,  flat  upon  the 
ground,  or  nearly  so,  and  more  or  less  overlapping,  is  very 
effectively  arranged  for  resisting  intense  light  or  drought 
or  cold  (see  Figs.  11,  12,  48). 

38.  Protective  positions. — In   other  cases,  a  position  is 
assumed  by  the  leaves  which  directs  their  flat 
surfaces  so   that   they  are   not  exposed  to  the 
most  intense  rays  of  light.     The  so-called  "  com- 


Pig.  41.  A  leaf  of  a  sensitive  plant  in  two  conditions.  In  the  figure  to  the  left  the 
leaf  is  fully  expanded,  with  its  four  main  divisions  and  numerous  leaflets  well 
spread.  In  the  figure  to  the  right  is  shown  the  same  leaf  after  it  has  been 
"shocked"  by  a  sudden  touch,  or  by  sudden  heat,  or  in  some  other  way.  The 
leaflets  have  been  thrown  together  forward  and  upward  ;  the  four  main  divisions 
have  been  moved  together ;  and  the  main  leaf-stalk  has  been  directed  sharply 
downward.  The  whole  change  has  very  much  reduced  the  surface  of  exposure. — 
After  Duchartbe. 


pass  plants/'  already  mentioned,  are  illustrations  of  this, 
the  leaves  standing  edgewise  and  receiving  on  their  surface 
the  less  intense  rays  of  light  (see  Figs.  5,  170).  In  the 
dry  regions  of  Australia  the  leaves  on  many  of  the  forest 
trees  and  shrubs  have  this  characteristic  edgewise  position, 
known  as  the  profile  position,  giving  to  the  foliage  a  very 
curious  appearance. 

Some  leaves  have  the  power  of  shifting  their  position 
according  to  their  needs,  directing  their  flat  surfaces  to- 
ward the  light,  or  more  or  less  inclining  them,  according 


Fie.  42.  The  telegraph  -p\mX(I)mnodium  gyrans).  Each  leaf  is  made  up  of  three 
leaflets,  a  large  terminal  one,  and  a  pair  of  small  lateral  ones.  In  the  lowest  figure 
the  large  leaflets  are  spread  out  in  their  day  position  ;  in  the  central  figure  they  are 
turned  sharply  downward  in  their  night  position.  The  name  of  the  plant  refers  to 
the  peculiar  and  constant  motion  of  the  pair  of  lateral  leaflets,  each  one  of  which 
describes  a  curve  with  a  jerking  motion,  like  the  second-hand  of  a  watch,  as 
indicated  in  the  uppermost  figure. 


50 


PLANT   RELATIONS. 


to  the  danger.  Perhaps  the  most  completely  adapted 
leaves  of  this  kind  are  those  of  the  "sensitive  plants/' 
whose  leaves  respond  to  various  external  influences  by 
changing  their  positions.  The  common  sensitive  plant 
abounds  in  dry  regions,  and  may  be  taken  as  a  type  of 
such  plants  (see  Figs.  4,  41,  171).  The  leaves  are  divided 
into  very  numerous  small  leaflets,  sometimes  very  small, 
which  stretch  in  pairs  along  the  leaf  branches.  When 
drought  approaches,  some  of  the  pairs  of  leaflets  fold  to- 
gether, slightly  reduc- 
ing the  surface  expo- 
sure. As  the  drought 
continues,  more  leaflets 
fold  together,  then  still 
others,  until  finally  all 
the  leaflets  may  be 
folded  together,  and  the 
leaves  themselves  may 
bend  against  the  stem. 
It  is  like  a  sailing  vessel 
gradually  taking  in  sail 
as  a  storm  approaches,  until  finally  nothing  is  exposed, 
and  the  vessel  weathers  the  storm  by  presenting  only  bare 
poles.  Sensitive  plants  can  thus  regulate  the  exposed  sur- 
face very  exactly  to  the  need. 

Such  motile  leaves  not  only  behave  in  this  manner  at  the 
coming  of  drought,  but  the  positions  of  the  leaflets  are 
shifted  throughout  the  day  in  reference  to  light,  and  at 
night  a  very  characteristic  position  is  assumed  (see  Figs.  2, 
3, 42),  once  called  a  "  sleeping  position."  One  danger  from 
night  exposure  may  come  from  the  radiation  of  heat  which 
might  chill  the  leaves  too  much ;  but  the  night  position 
may  have  no  such  meaning.  The  leaflets  of  Oxalis  have 
been  referred  to  (see  §14).  Similar  changes  in  the  direc- 
tion of  the  leaf  planes  at  the  coming  of  night  may  be 
observed  in  most  of  the  Leguminosce,  even  the  common 


Fio.  43.  Cotyledons  of  squash  seedling,  show- 
ing positions  in  light  (left  figure)  and  in 
darkness  (right  figure).— After  Atkinson. 


FOLIAGE   LEAVES  :    FUNCTION,    STRUCTURE,    ETC.        51 


white  clover  displaying  it.  It  can  be  observed  that  the 
expanded  seed  leaves  {cotyledons)  of  many  young  germinat- 
ing plants  shift  their  positions  at  night  (see  .Fig.  43),  often 
assuming  a  vertical  position  which  brings  them  in  contact 
with  one  another,  and  also  covers  the  stem  bud  (plumule). 

Certain  leaves  with  well-developed 
protective  structures  are  able  to  en- 
dure the  winter,  as  in  the  case  of 
the  so-called  evergreens.  In  the 
case  of  juniper,  however,  the  winter 
and  summer  positions  of  the  leaves 
are  quite  different  (see  Fig.  44).  In 
the  winter  the  leaves  lie  close  against 
the  stem  and  overlap  one  another; 
while  with  the  coming  of  summer 
conditions  they  become  widely 
spreading. 

39.  Protection  against  rain. — It  is 
also  necessary  for  leaves  to  avoid 
becoming  wet  by  rain.  If  the  water 
is  allowed  to  soak  in  there  is  danger 
of  filling  the  stomata  and  interfering 
with  the  air  exchanges.  Hence  it 
will  be  noticed  that  most  leaves  are 
able  to  shed  water,  partly  by  their 
positions,  partly  by  their  structure. 
In  many  plants  the  leaves  are  so  ar- 
ranged that  the  water  runs  off  towards  the  stem  and  so 
reaches  the  main  root  system  ;  in  other  plants  the  rain  is 
shed  outwards,  as  from  the  eaves  of  a  house. 

Some  of  the  structures  which  prevent  the  rain  from 
soaking  in  are  a  smooth  epidermis,  a  cuticle  layer,  waxy 
secretions,  felt-like  coverings,  etc.  Interesting  experi- 
ments may  be  performed  with  different  leaves  to  test  their 
power  of  shedding  water.  If  a  gentle  spray  of  water  is 
allowed  to  -play  upon  different  plants,  it  will  be  observed 


Fig.  44.  Two  twigs  of  juni 
per,  showing  the  ordi. 
nary  summer  and  winter 
positions  assumed  by  the 
leaves.  The  ordinary  pro- 
tected winter  position  of 
the  leaves  is  shown  by  A: 
while  in  B,  in  response  tn 
summer  conditions,  the 
leaves  have  spread  apart 
and  have  become  freelyex* 
.—After  Warming. 


52  PLANT  RELATIONS. 

that  the  water  glances  off  at  once  from  the  surfaces  of 
some  leaves,  runs  off  more  slowly  from  others,  and  may  be 
more  or  less  retained  by  others. 

In  this  same  connection  it  should  be  noticed  that  in 
most  horizontal  leaves  the  two  surfaces  differ  more  or  less 
in  appearance,  the  upper  usually  being  smoother  than  the 
lower,  and  the  stomata  occurring  in  larger  numbers,  some- 
times exclusively,  upon  the  under  surface.  While  these 
differences  doubtless  have  a  more  important  meaning  than 
protection  against  wetting,  they  are  also  suggestive  in  this 
connection. 


CHAPTER  IV. 

SHOOTS. 

40.  General  characters. — The  term  shoot  is  used  to  include 
both  stem  and  leaves.  Among  the  lower  plants,  such  as 
the  algse  and  toadstools,  there  is  no  distinct  stem  and  leaf. 
In  such  plants  the  working  body  is  spoken  of  as  the  thallus, 
which  does  the  work  done  by  both  stem  and  leaf  in  the 
higher  plants.  These  two  kinds  of  work  are  separated  in 
the  higher  plants,  and  the  shoot  is  differentiated  into  stem 
and  leaves. 

41.  Life-relation. — In  seeking  to  discover  the  essential 
life-relation  of  the  stem,  it  is  evident  that  it  is  not  neces- 
sarily a  light-relation,  as  in  the  case  of  the  foliage  leaf, 
for  many  stems  are  subterranean.  Also,  in  general,  the 
stem  is  not  an  expanded  organ,  as  is  the  ordinary  foli- 
age leaf.  This  indicates  that  whatever  may  be  its  essential 
life-relation  it  has  little  to  do  with  exposure  of  surface. 
It  becomes  plain  that  the  stem  is  the  great  leaf-bearing 
organ,  and  that  its  life-relation  is  a  leaf-relation.  Often 
stems  branch,  and  this  increases  their  power  of  producing 
leaves. 

In  classifying  stems,  therefore,  it  seems  natural  to  use 
the  kind  of  leaves  they  bear.  From  this  standpoint  there 
are  three  prominent  kinds  of  stems  :  (1)  those  bearing  foli- 
age leaves  ;  (2)  those  bearing  scale  leaves  ;  and  (3)  those 
bearing  floral  leaves.  There  are  some  peculiar  forms  of 
stems  which  do  not  bear  leaves  of  any  kind,  but  they  need 
not  be  included  in  this  general  view. 


54  PLANT  RELATIONS. 

A.     Stems  bearing  foliage  leaves. 

42.  General  character. — As  the  purpose  of  this  stem  is  to 
display  foliage  leaves,  and  as  it  has  been  discovered  that  the 
essential  life-relation  of  foliage  leaves  is  the  light-relation, 
it  follows  that  a  stem  of  this  type  must  be  able  to  relate  its 
leaves  to  light.  It  is,  therefore,  commonly  aerial,  and  that 
it  may  properly  display  the  leaves  it  is  generally  elongated, 
with  its  joints  {nodes)  bearing  the  leaves  well  separated  (see 
Figs.  1,  4,  18,  20). 

The  foliage-bearing  stem  is  generally  the  most  conspicu- 
ous part  of  the  plant  and  gives  style  to  the  whole  body. 
One's  impression  of  the  forms  of  most  plants  is  obtained 
from  the  foliage-bearing  stems.  Such  stems  have  great 
range  in  size  and  length  of  life,  from  minute  size  and  very 
short  life  to  huge  trees  which  may  endure  for  centuries. 
Branching  is  also  quite  a  feature  of  foliage-bearing  stems  ; 
and  when  it  occurs  it  is  evident  that  the  power  of  display- 
ing foliage  is  correspondingly  increased.  Certain  promi- 
nent types  of  foliage-bearing  stems  may  be  considered. 

43.  The  subterranean  type. — It  may  seem  strange  to  in- 
clude any  subterranean  stem  with  those  that  bear  foliage, 
as  such  a  stem  seems  to  be  away  from  any  light-relation. 
Ordinarily  subterranean  stems  send  foliage-bearing  branches 
above  the  surface,  and  such  stems  are  not  to  be  classed  as 
foliage-bearing  stems.  But  often  the  only  stem  possessed 
by  the  plant  is  subterranean,  and  no  branches  are  sent  to 
the  surface.  In  such  cases  only  foliage  leaves  appear  above 
ground,  and  they  come  directly  from  the  subterranean  stem. 
The  ordinary  ferns  furnish  a  conspicuous  illustration  of 
this  habit,  all  that  is  seen  of  them  above  ground  being  the 
characteristic  leaves,  the  commonly  called  "  stem  "  being 
only  the  petiole  of  the  leaf  (see  Figs.  45,  46,  144).  Many 
seed  plants  can  also  be  found  which  show  the  same  habit, 
especially  those  which  flower  early  in  the  spring.  This 
cannot  be   regarded  as  a  very  favorable  type  of  stem  for 


Fig.  45.  A  fern  (Agpidium),  showing  three  large  branching  leaves  coming  from  a  hori- 
zontal subterranean  stem  (rootstock) :  growing  leaves  are  also  shown,  which  are 
gradually  unrolling.  The  stem,  young  leaves,  and  petioles  of  the  large  leaves  are 
thickly  covered  with  protecting  hairs.  The  stem  gives  rise  to  numerous  small  roots 
from  its  lower  surface.  The  figure  marked  3  represents  the  under  surface  of  a 
portion  of  the  leaf,  showing  seven  groups  of  spore  cases ;  at  5  is  represented  a 
section  through  one  of  these  groups,  showing  how  the  spore  cases  are  attached  and 
protected  by  a  flap  ;  while  at  6  is  represented  a  single  spore  case  opening  and  dis- 
charging its  spores,  the  heavy  spring-like  ring  extending  along  the  back  and  over 
the  top.— After  Wossidlo. 


56  PLANT   RELATIONS. 

leaf  display,  and  as    a  rule  such   stems   do   not  produce 
many  foliage  leaves,  but  the  leaves  are  apt  to   be  large. 


Fig.  46.    A  common  fern,  showing  the  underground  stem  (rootstock),  which  sends  the 
few  large  foliage  leaves  above  the  surface.— After  Atkinson. 

The  subterranean  position  is  a  good  one,  however,  for 
purposes  of  protection  against  cold  or  drought,  and  when 
the  foliage  leaves  are  killed  new  ones  can  be  put  out  by 


SHOOTS. 


57 


the  protected  stem.  This  position  is  also  taken  advantage 
of  for  comparatively  safe  food  storage,  and  such  stems  are 
apt  to  become  more  or  less  thickened  and  distorted  by  this 
food  deposit. 

44.  The  procumbent  type. — In  this  case  the  main  body 
of  the  stem  lies  more  or  less  prostrate,  although  the  advanc- 
ing tip  is  usually  erect.  Such  stems  may  spread  in  all 
directions,  and  become  interwoven  into 
a  mat  or  carpet.  They  are  found 
especially  on  sterile  and  exposed  soil, 


Fig.  47.  A  strawberry  plant,  showing  a  runner  which  has  devel- 
oped a  new  plant,  which  in  turn  has  sent  out  another  run- 
ner.— After  Seubert. 


and  there  may  be  an  important  relation  between  this  fact  and 
their  habit,  as  there  may  not  be  sufficient  building  material 
for  erect  stems,  and  the  erect  position  might  result  in  too 
much  exposure  to  light,  or  heat,  or  wind,  etc.  Whatever 
may  be  the  cause  of  the  procumbent  habit,  it  has  its  advan- 
tages. As  compared  with  the  erect  stem,  there  is  economy 
of  building  material,  for  the  rigid  structures  to  enable  it  to 
stand  upright  are  not  necessary.  On  the  other  hand,  such 
a  stem  loses  in  its  power  to  display  leaves.  Instead  of 
being  free  to  put  out  its  leaves  in  every  direction,  one  side 
is  against  the  ground,  and  the  space  for  leaves  is  diminished 
at  least  one-half.  All  the  leaves  it  bears  are  necessarily 
directed  towards  the  free  side  (see  Fig.  18). 

We  may  be  sure,  however,  that  any  disadvantage  com- 
ing from  this  unfavorable  position  for  leaf  display  is  over- 
balanced by  advantages  in  other  respects.     The  position  is 


58 


PLANT  RELATIONS. 


certainly  one  of  protection,  and  it  has  a  further  advantage 
in  the  way  of  migration  and  vegetative  propagation.  As 
the  stem  advances  over  the  ground,  roots  strike  out  of  the 
nodes  into  the  soil.  In  this  way  fresh  anchorage  and  new 
soil  supplies  are  secured  ;  the  old  parts  of  the  stem  may 


i.  48.  Two  plants  of  a  saxifrage,  showing  rosette  habit,  and  also  the  numerous 
runners  sent  out  from  the  base,  which  strike  root  at  tip  and  produce  new  plants. 
— After  Kerner. 


die,  but  the  newer  portions  have  their  soil  connection  and 
continue  to  live.  80  effective  is  this  habit  for  this  kind  of 
propagation  that  plants  with  erect  stems  often  make  use  of 
it,  sending  out  from  near  the  base  special  prostrate  branches, 
which  advance  over  the  ground  and  form  new  plants. 
A  very  familiar  illustration  is  furnished  by  the  straw- 
berry plant,  which  sends  out  peculiar  naked  "runners" 
to  strike  root  and  form  new  plants,  which  then  become 


SHOOTS. 


59 


independent  plants  by  the  dying  of  the  runners  (see  Figs. 
47,  48). 

45.  The  floating  type. — In  this  case  the  stems  are  sus- 
tained by  water.  Numerous  illustrations  can  be  found  in 
small  inland  lakes  and  slow-moving  streams  (see  Fig.  49). 
Beneath  the  water  these  stems  often  seem  quite  erect,  but 


Fig.  49.    A  submerged  plant  {Ceratophyllum)  with  floating  stems,  showing 
joints  bearing  finely  divided  leaves. 


the  stem 


when  taken  out  they  collapse,  lacking  the  buoyant  powei 
of  the  water.  Growing  free  and  more  or  less  upright  in 
the  water,  they  seem  to  have  all  the  freedom  of  erect  sterna 
in  displaying  foliage  leaves,  and  at  the  same  time  they 
are  not  called  upon  to  build  rigid  structures.  Economy 
of  building  material  and  entire  freedom  to  display  foliage 
would  seem  to  be  a  happy  combination  for  plants.  It  must 
be  noticed,  however,  that  another  very  important  condition 
is  introduced.  To  reach  the  leaf  surfaces  the  light  must 
pass  through  the  water,  and  this  diminishes  its  intensity  so 


60 


PLANT   RELATIONS. 


greatly  that  the  working  power  of  the  leaves  is  reduced. 
At  no  very  great  depth  of  water  a  limit  is  reached,  beyond 
which  the  light  is  no  longer  able  to  be  of  service  to  the 
leaves  in  their  work.     Hence  it  is  that  water  plants  are 

restricted  to  the  surface  of  the 
water,  or  to  shoal  places ;  and  in 
such  places  vegetation  is  very 
abundant.  Water  is  so  serious 
an  impediment  to  light  that  very 
many  plants  bring  their  working 
leaves  to  the  surface  and  float 
them,  as  seen  in  water  lilies,  thus 
obtaining  light  of  undiminished 
intensity. 

46.  The  climbing  type. — Climb- 
ing stems  are  developed  especially 
in  the  tropics,  where  the  vegeta- 
tion is  so  dense  and  overshadow- 
ing that  many  stems  have  learned 
to  climb  upon  the  bodies  of  other 
plants,  and  so  spread  their  leaves 
in  better  light  (see  Figs.  50,  55, 
98,  212).  Great  woody  vines 
fairly  interlace  the  vegetation  of 
tropical  forests,  and  are  known 
as  "lianas/'  or  "lianes."  The 
same  habit  is  noticeable,  also,  in 
our  temperate  vegetation,  but  it 
is  by  no  means  bo  extensively  dis- 
played as  in  the  tropics.  There 
are  a  good  many  forms  of  climb- 
ing stems.  Remembering  that 
the  habit  refers  to  one  stem  de- 

Fio.  50.    A  vine  or  liana  climbing 

the  trunk  of  a  tree.    The  leaves     pending     upon     another     IOT 

are  ail  adjusted  to  face  the  light    mechanical  support,  we  may  in- 

and  to  avoid  shading  one  an-  , 

other  as  far  as  possible.  elude  many  hedge  plants  m  the 


SHOOTS.  61 

list  of  climbers.  In  this  case  the  stems  are  too  weak  to 
stand  alone,  but  by  interlacing  with  one  another  they  may 
keep  an  upright  position.  There  are  stems,  also,  which 
climb  by  twining  about  their  support,  as  the  hop  vine  and 


Fig.  51.    A  cluster  of  smilax,  showing  the  tendrils  which  enable  it  to  climb,  and  also 
the  prickles. — After  Kerner. 


morning  glory  ;  others  which  put  out  tendrils  to  grasp  the 
support  (see  Figs.  51,  52),  as  the  grapevine  and  star 
cucumber  ;  and  still  others  which  climb  by  sending  out 
suckers  to  act  as  holdfasts,  as  the  woodbine  (see  Figs.  53, 
54).    In  all  these  cases  there  is  an  attempt  to  reach  towards 


62  PLANT   RELATIONS. 

the  light  without  developing  such  structures  in  the  stem 
as  would  enable  it  to  stand  upright. 

47.  The   erect   type.— This  type   seems   altogether   the 
best  adapted  for  the  proper  display  of  foliage  leaves.   Leaves 


Fig.  52.  Passion-flower  vines  climbing  supports  by  means  of  tendrils,  which  may  be 
seen  more  or  less  extended  or  coiled.  The  two  types  of  leaves  upon  a  single  stem 
may  also  be  noted. 

can  be  sent  out  in  all  directions  and  carried  upward  to- 
wards the  light ;  but  it  is  at  the  expense  of  developing  an 
elaborate  mechanical  system  to  enable  the  stem  to  retain 
this  position.  There  is  an  interesting  relation  between 
these  erect  bodies  and  zones  of  temperature.     At  high  alti- 


SHOOTS. 


63 


Fig.  53.  Woodbine  (Ampelopsis)  in  a  deciduous  forest.  The  tree  trunks  are  almost 
covered  by  the  dense  masses  of  woodbine,  whose  leaves  are  adjusted  so  as  to  form 
compact  mosaics.  A  lower  stratum  of  vegetation  is  visible,  composed  of  shrubs 
and  tall  herbs,  showing  that  the  forest  is  somewhat  open. — After  ScHistrER. 


tudes  or  latitudes  the  subter- 
ranean and  prostrate  types  of 
foliage-bearing  stems  are  most 
common  ;  and  as  one  passes  to 
lower  altitudes  or  latitudes  the 
erect  stems  become  more  nu- 
merous and  more  lofty.  Among 
stems  of  the  erect  type  the  tree 
is  the  most  impressive,  and  it 
has  developed  into  a  great  vari- 
ety of  forms  or  "habits."  Any 
one  recognizes  the  great  differ- 
ence in  the  habits  of  the  pine 
and  the  elm  (see  Figs.  56, 
57,   58,   59),   and  many  of  our 


Fig.  54.  A  portion  of  a  woodbine 
{Ampelopsis).  The  stem  tendrils 
have  attached  themselves  to  a 
smooth  wall  by  means  of  disk-like 
suckers. — After  Strasburger. 


Fig.  55.    A  liana  in  the  Botanic  Garden  at  Peradenyia,  Ceylon. — After  Schihper. 


Fig.  5$.  A  tree  of  the  pine  type  (larch),  showing  the  continuous  central  shaft  and 
the  horizontal  branches,  which  tend  to  become  more  upright  towards  the  top  of 
the  tree.  The  general  outline  is  distinctly  conical.  The  larch  is  peculiar  among 
such  trees  in  periodically  shedding  its  leaves 


Fig.  57.  A  pine  tree,  showing  the  central  shaft  and  also  the  bunching  of  the 
needle  leaves  toward  the  tips  of  the  branches  where  there  is  the  best  exposure 
to  light. 


SHOOTS. 


67 


common  trees  may  be  known,  even  at  a  distance,  by  their 
characteristic  habits  (see  Figs.  60,  01,  G2).  The  difficulty 
of  the  mechanical  problems  solved  by  these  huge  bodies 
is  very  great.  They  maintain  form  and  position  and  en- 
dure tremendous  pressure  and  strain. 


Fig.  58.  An  elm  in  its  winter  condition,  showing  the  absence  of  a  continuous  central 
shaft,  the  main  stem  soon  breaking  up  into  branches,  and  giving  a  spreading  top 
On  each  side  in  the  background  are  trees  of  the  pine  type,  showing  the  central 
shaft  and  conical  outline. 


68  PLANT   RELATIONS. 

48.  Relation  to  light. — As  stems  bearing  foliage  leaves 
hold  a  special  relation  to  light,  it  is  necessary  to  speak  of 
the  influence  of  light  upon  their  direction,  the  response  to 


Fig.  59.    An  elm  in  foliage,  showing  the  breaking  up  of  the  trunk  into  branches  and 
the  spreading  top. 

which  is  known  as  heliotropism,  already  referred  to  under 
foliage  leaves.  In  the  case  of  an  erect  stem  the  tendency 
is  to  grow  towards  the  source  of  light  (see  Figs.  1,  64). 


SHOOTS.  69 

This  has  the  general  result  of  placing  the  leaf  blades  at 
right  angles  to  the  rays  of  light,  and  in  this  respect  the 
heliotropism  of  the  stem  aids  in  securing  a  favorable  leaf 
position  (see  Figs.  63,  63a).  Prostrate  stems  are  differently- 
affected  by  the  light,  however,  being  directed  transversely 
to  the  rays  of  light.     The  same  is  true  of  many  foliage 


■ 

F  ■■  .Ayv  v  ^i^Sj~i^=^ 

\kir 

1 

?WBfcZ    *  <awK  S^r^         y  j^   if  t'<?5C^*>^S'^ 

"^^^^^X^ki 

M 

Fig.  60.    An  oak  in  its  winter  condition,  showing  the  wide  branching.    The  various 
directions  of  the  branches  have  been  determined  by  the  light-relations. 


branches,  as  may  be  seen  by  observing  almost  any  tree  in 
which  the  lower  branches  are  in  the  general  transverse  posi- 
tion. These  branches  generally  tend  to  turn  upwards  when 
they  are  beyond  the  region  of  shading.  Subterranean  stems 
are  also  mostly  horizontal,  but  they  are  out  of  the  influence 
of  light,  and  under  the  influence  of  gravity,  the  response  to 
which  is  known  as  geotropism,  which  guides  them  into  the 
transverse  position.    The  climbing  stem,  like  the  erect  one, 


70 


PLANT   RELATIONS. 


Fig.  61.    Cottonwoods,  in  winter  condition,  on  a  sand  dune,  showing  the  branching 
habit,  and  the  tendency  to  grow  in  groups. 


grows   towards   the   light,   while   floating   stems   may    be 
either  erect  or  transverse. 


B.     Stems  hearing  scale  leaves. 

49.  General  character. — A  scale  leaf  is  one  which  does 
not  serve  as  foliage,  as  it  does  not  develop  the  necessary 
chlorophyll.  This  means  that  it  does  not  need  such  an 
exposure  of  surface,  and  hence  scale  leaves  are  usually  much 
smaller,  and  certainly  are  more  inconspicuous  than  foliage 
leaves.  A  good  illustration  of  scale  leaves  is  furnished  by 
the  ordinary  scaly  buds  of  trees,  in  which  the  covering  of 
overlapping  scaly  leaves  is  very  conspicuous  (see  Fig.  G5). 
As  there  is  no  development  of  chlorophyll  in  such  leaves, 


SHOOTS. 


71 


they  do  not  need  to  be  exposed  to  the  light.  Stems  bearing 
only  scale  leaves,  therefore,  hold  no  necessary  light-relation, 
and  may  be  subterranean  as  well  as  aerial.     For  the  same 


h'lo.  62.  A  group  of  weeping  birches,  showing  the  branching  habit  and  the  peculiar 
hanging  branchlets.  The  trunks  also  show  the  habit  of  birch  bark  in  peeling  off 
in  bands  around  the  stem. 


reason  scale  leaves  do  not  need  to  be  separated  from  one 
another,  but  may  overlap,  as  in  the  buds  referred  to. 

Sometimes  scale  leaves  occur  so  intermixed  with  foliage 


Fig.  63. 


Sunflowers  with  the  upper  part  of  the  stem  eharpJy  bent  towards  the  light, 
giving  the  leaves  better  exposure.— After  Sch affner. 


SHOOTS. 


73 


leaves  that  no  peculiar  stem  type  is  developed.  In  the 
pines  scale  leaves  are  found  abundantly  on  the  stems  which 
are  developed  for  foliage  purposes.  In  fact,  the  main  stem 
axes  of  pines  bear  only  scale  leaves,  while  short  spur-like 
branches  bear  the  characteristic  needles,  or  foliage  leaves, 
but  the  form  of  the 
stem  is  controlled 
by  the  needs  of  the 
foliage.  Some  very 
distinct  types  of 
scale-bearing  stems 
may  be  noted. 

50.  The  bud  type. 
— In  this  case  the 
nodes  bearing  the 
leaves  remain  close 
together,  not  sepa- 
rating, as  is  neces- 
sary in  ordinary 
foliage-bearing 
stems,  and  the 
leaves  overlap.  In 
a  stem  of  this  char- 
acter the  later  joints 
may  become  sepa- 
rated and  bear  foli- 
age leaves,  so  that 
one  finds  scale  leaves 
the  same  stem  axis. 


Fig.  63a.  Cotyledons  of  castor-oil  bean  ;  the  seedling 
to  the  left  showing  the  ordinary  position  of  the 
cotyledons,  the  one  to  the  right  showing  the  curva- 
ture of  the  stem  in  response  to  light  from  one 
side. — After  Atkinson. 


below  and  foliage  leaves  above  on 
This  is  always  true  in  the  case  of 
branch  buds,  in  which  the  scale  leaves  serve  the  purpose 
of  protection,  and  are  aerial,  not  because  they  need  a 
light-relation,  but  because  they  are  protecting  young  foli- 
age leaves  which  do. 

Sometimes  the  scale  leaves  of  this  bud  type  of  stem  do 
not  serve  so  much  for  protection  as  for  food  storage,  and 
become  fleshy.     Ordinary  bulbs,  such  as  those  of  lilies,  etc., 


74 


PLANT   RELATIONS. 


Fig.  64.  An  araucarian  pine,  showing  the 
central  shaft,  and  the  regular  clusters  of 
branches  spreading  in  every  direction  and 
bearing  numerous  small  leaves.  The  low- 
ermost branches  extend  downwards  and 
are  the  largest,  while  those  above  become 
more  horizontal  and  smaller.  These  dif- 
ferences in  the  size  and  direction  of  the 
branches  secure  the  largest  light  expo- 
sure. 


are  of  this  character ; 
and  as  the  main  pur- 
pose is  food  storage 
the  most  favorable 
position  is  a  subter- 
ranean one  (see  Fig. 
66).  Sometimes  such 
scale  leaves  become 
very  broad  and  not 
merely  overlap  but  en- 
wrap one  another,  as 
in  the  case  of  the 
onion. 

51.  The  tuber  type. 
— The  ordinary  potato 
may  be  taken  as  an  il- 
lustration (see  Fig. 
07).  The  minute  scale 
leaves,  to  be  found  at 
the  "eyes"  of  the 
potato,  do  not  overlap, 
which  means  that  the 
stem  joints  are  farther 
apart  than  in  the  bud 
type.  The  whole  form 
of  the  stem  results 
from  its  use  as  a  place 
of  food  storage,  and 
hence  such  stems  are 
generally  subterra- 
nean. Food  storage, 
subterranean  position, 
and  reduced  scale 
leaves  are  facts  which 
seem  to  follow  each 
other  naturally. 


SHOOTS. 


75 


52.  The  rootstock  type. — This  is  prob- 
ably the  most  common  form  of  subter- 
ranean stem.  It  is  elongated,  as  are  foli- 
age stems,  and  hence  the  scale  leaves 
are  well  separated.  It  is  prominently 
used  for  food  storage,  and  is  also  admirably 
adapted  for  subterranean  migration  (see 
Fig.  68).  It  can  do  for  the  plant,  in  the 
way  of  migration,  what  prostrate  foliage- 
bearing  stems  do,  and  is  in  a  more  protected 
position.  Advancing  beneath  the  ground, 
it  sends  up  a  succession  of  branches 
to  the  surface.  It  is  a  very  efficient 
method  for  the  ''spreading"  of  plants, 
and  is  extensively  used  by  grasses  in  cov- 
ering areas  and  forming  turf.  The  persist- 
ent continuance  of  the  worst  weeds  is  often 
due  to  this  habit  (see  Figs.  69,  70).     It 

is  impossible 


Fig.  65.  Branch  buds 
of  elm.  Three  buds 
(£)  with  their  over- 
lapping scales  are 
shown,  each  just 
above  the  scar  (b) 
of  an  old  leaf. — 
After  Behrens. 


Fig.  66.  A  bulb,  made  up  of  overlap- 
ping scales,  which  are  fleshy  on 
account  of  food  storage.  —  After 
Gray. 


to    remove 

all    of    the 

indefinitely 

branching 

rootstocks 

from  the  soil, 

and  any  fragments  that  remain 

are  able  to  send  up  fresh  crops 

of  aerial  branches. 

53.  Alternation  of  rest  and 
activity. — In  all  of  the  three 
stem  types  just  mentioned,  it 
is  important  to  note  that  they 
are  associated  with  a  remark- 
able alternation  between  rest 
and  vigorous  activity.  From 
the  branch  buds  the  new  leaves 


76 


PLANT  RELATIONS. 


emerge  with 
great  rapidity, 
and  trees  be- 
come covered 
with  new  foliage 
in  a  few  days. 
From  the  sub- 
terranean stems 
the  aerial  parts 
come  up  so 
speedily  that  the 
surface  of  the 
ground  seems  to 
be  covered  suddenly  with  young  vegetation.  This  sudden 
change  from  comparative  rest  to  great  activity  has  been 
well  spok*en  of  as  the  "  awakening  "  of  vegetation. 


Fig.  67.    A  potato  plant,  showing  the  subterranean  tubers.— 
After  Strasbubger. 


C.     Stems  bearing  floral  leaves. 


54.  The  flower. — The  so-called 
"flowers"  which  certain  plants 
produce  represent  another  type  of 
shoot,  being  stems  with  peculiar 
leaves.  So  attractive  are  flowers 
that  they  have  been  very  much 
studied ;  and  this  fact  has  led 
many  people  to  believe  that  flowers 
are  the  only  parts  of  plants  worth 
studying.  Aside  from  the  fact 
that  a  great  many  plants  do  not 
produce  flowers,  even  in  those 
that  do  the  flowers  are  connected 
with  only  one  of  the  plant  pro- 
cesses, that  of  reproduction. 
Every  one  knows  that  flowers  are 
exceedingly   variable,  and   names 


Fig.  68.  The  rootstock  of  Solo- 
mon's seal ;  from  the  under  side 
roots  are  developed  ;  and  on  the 
upper  side  are  seen  the  scars 
which  mark  the  positions  of  the 
successive  aerial  branches  which 
bear  the  leaves.  The  advanc- 
ing tip  is  protected  by  scales 
(forming  a  bud),  and  the  posi- 
tions of  previous  buds  are  in- 
dicated by  groups  of  ring-like 
scars  which  mark  the  attach- 
ment of  former  scales.  Advanc- 
ing in  front  and  dying  behind 
such  a  rootetock  may  give  rise 
to  an  indefinite  succession  of 
aerial  plants.— After  Gray. 


SHOOTS. 


77 


have  been  given  to  every  kind  of  variation,  so  that  their 
study  is  often  not  much  more  than  learning  the  definitions 
of  names.  However,  if  we  seek  to  discover  the  life-rela- 
tions of  flowers  we  find  that  they  may  be  stated  very  simply. 
55.  Life-relations. — The  flower  is  to  produce  seed.  It 
must  not  only  put  itself  into  proper  relation  to  do  this,  but 


Fig.  69.  The  rootstock  of  a  rash  (Juncus),  showing  how  it  advances  beneath  the 
ground  and  sends  above  the  surface  a  succession  of  branches.  The  breaking  up 
of  such  a  rootstock  only  results  in  so  many  separate  individuals.— After  Cowles. 

there  must  also  be  some  arrangement  for  putting  the  seeds 
into  proper  conditions  for  developing  new  plants.  In  the 
production  of  seed  it  is  necessary  for  the  flower  to  secure  a 
transfer  of  certain  yellowish,  powdery  bodies  which  it  pro- 
duces, known  as  pollen  or  pollen-grains,  to  the  organ  in 
which  the  seeds  are  produced,  known  as  the  pistil.  This 
transfer  is  called  pollination.  One  of  the  important  things, 
therefore,  in  connection  with  the  flower,  is  for  it  to  put 


78 


PLANT   RELATIONS. 


Fig.  70.    An  alpine  willow,  showing  a  strong  rootstock  developing  aerial  branches 
and  roots,  and  capable  of  long  life  and  extensive  migration.— After  Schimper. 

itself  into  such  relations  that  it  may  secure  pollination. 

Besides  pollination, 
which  is  necessary 
to  the  production  of 
seeds,  there  must  be 
an  arrangement  for 
seed  dispersal.  It 
is  always  well  for 
seeds  to  be  scattered, 
so  as  to  be  separated 
from  one  another 
and  from  the  parent 
plant.  The  two 
great  external  prob- 

Fig.  71.    A  flower  of  peony,  showing  the  four  sets  of  °  t  x    m 

floral  organs  :    k,  the  sepals,  together  called  the  lems     ill     Connection 

calyx  ;  c,  the  petals,  together  called  the  corolla  ;  with     the     flower 
a,  the  numerous  stamens;   g,  the  two  carpels,  . 

which  contain  the  ovules.— After  Strasburger.  therefore,    are  polll- 


SHOOTS. 


79 


nation  and  seed-dispersal.  It 
is  necessary  to  call  attention 
to  certain  peculiar  features  of 
this  type  of  stem. 

56.  Structures. — The  joints 
of  the  stem  do  not  spread 
apart,  so  that  the  peculiar 
leaves  are  kept  close  together, 
usually  forming  a  rosette-like 
cluster  (see  Fig.  71).  These 
leaves  are  of  four  kinds  :  the 
lowest  (outermost)  ones  (indi- 
vidually sepals,  collectively 
calyx)  mostly  resemble  small 
foliage  leaves  ;  the  next  higher 
(inner)  set  (individually petals, 
collectively  corolla)  are  usually 
the  most  conspicuous,  delicate 
in  texture  and  brightly  col- 
ored ;  the  third  set  (stamens) 
produces  the  pollen  ;  the 
highest  (innermost)  set  (car- 
pels) form  the  pistil  and  pro- 
duce the  ovules,  which  are  to 
become  seeds.  These  four  sets 
may  not  all  be  present  in  the 
same  flower  ;  the  members  of 
the  same  set  may  be  more  or 
less  blended  with  one  another, 
forming  tubes,  urns,  etc.  (see 
Figs.  72,  73,  74)  ;  or  the  dif- 
ferent members  may  be  modi- 
fied in  the  greatest  variety  of 
ways. 

Another  peculiarity  of  this 
type  of  stem  is  that  when  the 


Fig.  72.  A  group  of  flowers  of  the  rose 
family.  The  one  at  the  top  (Polen- 
tilla)  shows  three  broad  sepals, 
much  smaller  petals  alternating 
with  them,  a  group  of  stamens,  and 
a  large  receptacle  bearing  numer- 
ous small  carpels.  The  central  one 
{Alchemilla)  shows  the  tips  of  two 
small  sepals,  three  larger  petals 
united  below,  stamens  arising  from 
the  rim  of  the  urn,  and  a  single  pe- 
culiar pistil.  The  lowest  flower  (the 
common  apple)  shows  the  sepals, 
petals,  stamens,  and  three  styles, 
all  arising  from  the  ovary  part  of 
the  pistil.— After  Focke. 


80 


PLANT   RELATIONS. 


Pit;.  73.  A  flower  of  the  tobacco  plant :  a,  a  complete  flower,  showing  the  calyx  with 
its  sepals  blended  below,  the  funnelforni  corolla  made  up  of  united  petals,  and  the 
stamens  just  showing  at  the  mouth  of  the  corolla  tube  ;  b,  acorolla  tube  split  open 
and  showing  the  five  stamens  attached  to  it  near  the  base  ;  c,  a  pistil  made  up  of 
two  blended  carpels,  the  bulbous  base  (containing  the  ovules)  being  the  ovary,  the 
long  stalk-like  portion  the  style,  and  the  knob  at  the  top  the  stigma.— After 
Strasburger. 


last  set  of  floral  leaves  {carpels)  appear,  the  growth  of  the 
stem  in  length  is  checked  and  the  cluster  of  floral  leaves 


n  b  c  d  e 

Pig.  7i.  A  group  of  flower  forms:  a,  a  flower  of  harebell,  showing  a  bell-shaped 
corolla  composed  of  five  petals  ;  b,  a  flower  of  phlox,  showing  a  tubular  corolla 
with  its  five  petals  distinct  above  and  sharply  spreading  ;  c,  a  flower  of  dead-nettle, 
showing  an  irregular  corolla  with  its  five  petals  forming  two  lips  above  the  funnel- 
form  base  ;  d,  a  flower  of  toad-flax,  showing  a  two-lipped  corolla,  and  also  a  spur 
formed  by  the  base  of  the  corolla  ;  e,  a  flower  of  the  snapdragon,  showing  the  two 
lips  of  the  corolla  closed.— After  Gray. 


SHOOTS.  81 

appears  to  be  upon  the  end  of  the  stem  axis.     It  is  usual, 
also,  for  the  short  stem  bearing  the  floral  leaves  to  broaden 


Fig.  75.  The  Star-of -Bethlehem  (Ornilhogalum),  showing  the  loose  cluster  of  flowers 
at  the  end  of  the  stem.  The  leaves  and  stem  arise  from  a  bulb,  which  produces  a 
cluster  of  roots  below.— After  Strasburgek. 

at  the  apex  and  form  what  is  called  a  receptacle,  upon  which 
the  close  set  floral  leaves  stand. 

Although  many  floral  stems  are  produced  singly,  it  is 


82  PLANT   RELATIONS. 

very  common  for  them  to  branch,  so  that  the  flowers  appear 
in  clusters,  sometimes  loose  and  spray-like,  sometimes  com- 
pact  (see  Figs.   75,   76,   77).     For  example,  the  common 


Fig.  76.    A  flower  cluster  from  a  walnut  tree.— After  Strasburger. 

dandelion  "flower"  is  really  a  compact  head  of  flowers. 
All  of  this  branching  has  in  view  better  arrangements  for 
pollination  or  for  seed-distribution,  or  for  both. 

The  subject  of  pollination  and  seed-distribution  will  be 
considered  under  the  head  of  reproduction. 


SHOOTS. 


83 


STRUCTURE    AND    FUNCTION"    OF   THE    STEM. 

57.  Stem  structure. — The  aerial  foliage  stem  is  the  most 
favorable  for  studying  stem  structure,  as  it  is  not  distorted 
by  its  position  or  by  being  a  depository  for  food.  If  an 
active  twig  of  an  ordinary  woody  plant  be  cut  across,  it  will 


Fig. 


Flower  clusters  of  an  umbellifer  (Sium).— After  Strasburger. 


be  seen  that  it  is  made  up  of  four  general  regions  (see  Fig. 
78):  (1)  an  outer  protecting  layer,  which  may  be  stripped 
off  as  a  thin  skin,  the  epidermis  ;  (2)  within  the  epider- 
mis a  zone,  generally  green,  the  cortex  ;  (3)  an  inner  zone 
of  wood  or  vessels,  known  as  the  vascular  region ;  (4)  a 
central  pith. 

58.  Dicotyledons  and  Conifers. — Sometimes  the  vessels 
7 


84 


PLANT  RELATIONS. 


Fig.  78.  Section  across  a  young  twig  of  box 
elder,  showing  the  four  stem  regions :  e, 
epidermis,  represented  by  the  heavy  bound- 
ing line  ;  c,  cortex  ;  w,  vascular  cylinder  ; 
p,  pith. 


are  arranged  in  a  hollow 
cylinder,  just  inside  of 
the  cortex,  leaving  what 
is  called  pith  in  the 
center  (see  Fig.  78). 
Sometimes  the  pith  dis- 
appears in  older  stems  or 
parts  of  stems  and  leaves 
the  stem  hollow.  When 
the  vessels  are  arranged 
in  this  way  and  the  stem 
lives  more  than  a  year,  it 
can  increase  in  diameter 
by  adding  new  vessels 
outside   of    the   old.     In 

the  case  of  trees  these  additions  appear  in  cross-section  like 

a  series  of  concentric  rings,  and  as  there  is  usually  but  one 

growth  period  during  the  year,  they  are  often  called  annual 

rings  (see  Fig.  79),  and  the  age  of  a  tree  is  often  estimated 

by    counting    them. 

This  method  of  ascer- 
taining  the   age   of   a 

tree  is  not  absolutely 

certain,   as  there  may 

be    more   than    one 

growth  period  in  some 

years.     In  the  case  of 

trees   and   shrubs   the 

epidermis   is    replaced 

on  the  older  parts  by 

layers  of   cork,  which 

sometimes    becomes 

verv  thick  and  makes     FlG-  79#  Section  across  a  twig  of  box  eider  three 

years  old,  showing  three  annual  rings,  or  growth 

Up    tne     Outer    part    01  rings,  in  the  vascular  cylinder.    The  radiating 

what      is      COmmonlv  lines  (»»)  which  cross  the  vascular  region  (w)  rep. 

, .     .    ,       .  resent  the  pith  rays,  the  principal  ones  extending 

Called    bark.  from  the  pith  to  the  cortex  (c). 


SHOOTS. 


85 


Stems  which  increase  in  diameter  mostly  belong  to  the 
great  groups  called  Dicotyledons  and  Conifers.  To  the 
former  belong  most  of  our  common  trees,  such  as  maple, 
oak,  beech,  hickory,  etc.  (see  Figs.  58,  59,  60,  61),  as 
well  as  the  great  majority  of  common  herbs;  to  the  latter 
belong  the  pines,  hemlocks,  etc.  (see  Figs.  56,  57,  198 
to  201).  This  annual  increase  in  diameter  enables  the 
tree  to  put  out  an  increased  number  of  branches  and 
hence  foliage  leaves  each  year,  so 
that  its  capacity  for  leaf  work  be- 
comes greater  year  after  year.  A 
reason  for  this  is  that  the  stem  is 
conducting  important  food  sup- 
plies to  the  leaves,  and  if  it  in- 
creases in  diameter  it  can  conduct 
more  supplies  each  year  and  give 
work  to  more  leaves. 

59.  Monocotyledons. — In  other 
stems,  however,  the  vessels  are 
arranged  differently  in  the  central 
region.  Instead  of  forming  a  hol- 
low cylinder  enclosing  a  pith,  they 
are  scattered  through  the  central 
region,  as  may  be  seen  in  the  cross- 
section  of  a  corn-stalk    (see  Fig. 

80).  Such  stems  belong  mostly  to  a  great  group  of  plants 
known  as  Monocotyledons,  to  which  belong  palms,  grasses, 
lilies,  etc.  For  the  most  part  such  stems  do  not  increase  in 
diameter,  hence  there  is  no  branching  and  no  increased 
foliage  from  year  to  year.  A  palm  well  illustrates  this 
habit,  with  its  columnar,  unbranching  trunk,  and  its  crown 
of  foliage  leaves,  which  are  about  the  same  in  number  from 
year  to  year  (see  Figs.  81,  82). 

60.  Ferns. — The  same  is  true  of  the  stems  of  most  fern- 
plants,  as  the  vessels  of  the  central  region  are  so  arranged 
that  there  can  be  no  diameter  increase,  though  the  ar- 


Fig.  80.  A  corn-stalk,  showing 
cross-section  and  longitudinal 
section.  The  dots  represent 
the  scattered  bundles  of  ves- 
sels, which  in  the  longitudinal 
section  are  seen  to  be  long 
fiber-like  strands. 


Fig.  81.  A  date  palm,  showing  the  unbranched  columnar  trunk  covered  with  old  leaf 
bases,  and  with  a  cluster  of  huge  active  leaves  at  the  top,  only  the  lowest  portions 
of  which  are  shown.    Two  of  the  very  heavy  fruit  clusters  are  also  shown. 


SHOOTS. 


87 


rangement  is  very  different  from  that  found  in  Monocotyle- 
dons. It  will  be  noticed  Iioav  similar  in  general  appearance 
is  the  habit  of  the  tree  fern  and  that  of  the  palm  (see  Fig. 
83). 

61.  Lower  plants. — In  the  cuse  of  moss-plants,  and  such 
algse  and  fungi  as  develop  stems,  the  stems  are  very  much 


Fig.  82.    A  palm  of  the  palmetto  type  (fan  palm),  with  low  stem  and  a  crown  of  large 

leaves. 


simpler  in   construction,  but  they  serve  the  same  general 
purpose. 

62.  Conduction  by  the  stem. — Aside  from  the  work  of 
producing  leaves  and  furnishing  mechanical  support,  the 
stem  is  a  great  conducting  region  of  the  plant.  This  sub- 
ject will  be  considered  in  Chapter  X.,  under  the  general 
head  of  "The  Nutrition  of  Plants." 


Pig.  83.  A  group  of  tropical  plants.  To  the  left  of  the  center  is  a  tree  fern,  with  its 
Blender  columnar  stem  and  crown  of  large  leaves.  The  large-leaved  plants  to  the 
right  are  bananas  (monocotyledons). 


CHAPTER  V. 

ROOTS. 

63.  General  character. — The  root  is  a  third  prominent 
plant  organ,  and  it  presents  even  a  greater  variety  of  rela- 
tions than  leaf  or  stem.  In  whatever  relation  it  is  found 
it  is  either  an  absorbent  organ  or  a  holdfast,  and  very  often 
both.  For  such  work  no  light-relation  is  necessary,  as  in 
the  case  of  foliage  leaves  ;  and  there  is  no  leaf-relation,  as 
in  the  case  of  stems.  Roots  related  to  the  soil  may  be 
taken  as  an  illustration. 

It  is  evident  that  a  soil  root  anchors  the  plant  in  the 
soil,  and  also  absorbs  water  from  the  soil.  If  absorption  is 
considered,  it  is  further  evident  that  the  amount  of  it  will 
depend  in  some  measure  upon  the  amount  of  surface  which 
the  roots  expose  to  the  soil.  We  have  already  noticed  that 
the  foliage  leaf  has  the  same  problem  of  exposure,  and  it 
solves  it  by  becoming  an  expanded  organ.  The  question 
may  be  fairly  asked,  therefore,  why  are  not  roots  expanded 
organs  ?  The  receiving  of  rays  of  light,  and  the  absorbing 
of  water  are  very  different  in  their  demands.  In  the  former 
case  a  flat  surface  is  demanded,  in  the  latter  tubular  pro- 
cesses. The  increase  of  surface  in  the  root,  therefore,  is 
obtained  not  by  expanding  the  organ,  but  by  multiplying 
it.  Besides,  to  obtain  the  soil  water  the  roots  must  burrow 
in  every  direction,  and  must  send  out  their  delicate  thread- 
like branches  to  come  in  contact  with  as  much  soil  as  pos- 
sible. Furthermore,  in  soil  roots  absorption  is  not  the  only 
thing  to  consider,  for  the  roots  act  as  holdfasts  and  must 
grapple  the  soil.     This  is  certainly  done  far  more  effectively 


90 


PLANT   RELATIONS. 


by  numerous  thread-like  processes  spreading  in  every  direc- 
tion than  by  flat,  expanded  processes. 

It  should  also  be  noted  that  as  soil  roots  are  subterra- 
nean they  are  used  often  for  the  storage  of  food,  as  in  the 
case  of  many  subterranean  stems.  Certain  prominent  root 
types  may  be  noted  as  follows  : 

64.  Soil  roots. — These  roots  push  into  the  ground  with 

great  energy, 
and  their  ab- 
sorbing sur- 
faces are  en- 
tirely covered. 
Only  the  young- 
est parts  of  a 
root  system 
absorb  actively, 
the  older  parts 
transporting 
the  absorbed 
material  to  the 
stem,  and  help- 
ing to  grip  the 
soil.  The  soil 
root  is  the  most 
common  root 
type,  being 
used  by  the  great  majority  of  seed  plants  and  fern  plants, 
and  among  the  moss  plants  the  very  simple  root-like  pro- 
cesses are  mostly  soil-related.  To  such  roots  the  water  of 
the  soil  presents  itself  either  as  free  water — that  is,  water 
that  can  be  drained  away — or  as  films  of  water  adhering  to 
each  soil  particle,  often  called  water  of  adhesion.  To  come 
in  contact  with  this  water,  not  only  does  the  root  system 
usually  branch  profusely  in  every  direction,  but  the  youngest 
branches  develop  abundant  absorbing  hairs,  or  root  hairs 
(see  Fig.  84),  which  crowd  in  among  the  soil  particles  and 


Fig.  84.  Root  tips  of  corn,  showing  root  hairs  and  their 
position  in  reference  to  the  growing  tip :  1,  in  soil  (higher 
up  the  hair6  become  much  more  abundant  and  longer) ; 
2,  in  moist  air. 


KOOTS. 


91 


absorb  moisture  from  them. 


Fig.  85.  Apparatus  to  show  the  influence 
of  water  (hydrotropism)  upon  the  direc- 
tion of  roots.  The  ends  (a)  of  the  box 
have  hooks  for  hanging,  while  the  box 
proper  is  a  cylinder  or  trough  of  wire 
netting  and  is  filled  with  damp  sawdust. 
In  the  sawdust  are  planted  peas  (g), 
whose  roots  (h.  i,  fc,  m)  first  descend  until 
they  emerge  from  the  damp  sawdust,  but 
soon  turn  back  toward  it. — After  Sachs. 


By  these  root  hairs  the  ab- 
sorbing surface,  and  hence 
the  amount  of  absorption, 
is  greatly  increased.  Indi- 
vidual root  hairs  do  not  last 
very  long,  but  new  ones  are 
constantly  appearing  just 
behind  the  advancing  root 
tips,  and  the  old  ones  are 
as  constantly  disappearing. 
(1)  Geotropism  and  hy- 
drotropism.— Many  outside 
influences  affect  roots  in 
the  direction  of  their 
growth,  and  as  soil  roots 
are  especially  favorable  for 
observing  these  influences, 
two  prominent  ones  may 
be  mentioned.  The  influ- 
ence of  gravity,  or  the  earth 
influence,  is  very  strong 
in  directing  the  soil   root. 


Fig.  86. 


A  raspberry  plant,  whose  stem  has  been  bent  down  to  the  soil  and  has 
"struck  root."— After  Beal. 


92  PLANT   RELATIONS. 

As  is  well  known,  when  a  seed  germinates  the  tip  that  is  to 
develop  the  root  turns  towards  the  earth,  even  if  it  has 
come  from  the  seed  in  some  other  direction.  This  response 
to  gravity  by  the  plant  is  known  as  geotropism.  Another 
directing  influence  is  moisture,  the  response  to  which  is 


Fig.  87.  A  section  through  the  leaf-stalk  of  a  yellow  pond-lily  (Nuphar),  showing  the 
numerous  conspicuous  air  passages  (s)  by  means  of  which  the  parts  under  water 
are  aerated ;  h,  internal  hairs  projecting  into  the  air  passages ;  v,  the  much 
reduced  and  comparatively  few  vascular  bundles. 

known  as  hydrotropism.  By  means  of  this  the  root  is  di- 
rected towards  the  most  favorable  water  supply  in  the  soil. 
Ordinarily,  geotropism  and  hydrotropism  direct  the  root 
in  the  same  general  way,  and  so  reinforce  each  other ;  but 
the  following  experiment  may  be  arranged,  which  will 
separate  these  two  influences.  Bore  several  small  holes  in 
the  bottom  of  a  box,  suspended  as  indicated  in  Fig.  85, 
and  cover  the  bottom  and  surround  the  box  with  blotting 
paper.     Pass  the  root  tips  of  several  germinated   seeds 


ROOTS. 


93 


through  the  holes,  so  that  the  seeds  rest  on  the  paper,  and 
the  root  tips  hang  through  the  holes.  If  the  paper  is  kept 
moist  germination  will  continue,  but  geotropism  will  direct 
the  root  tips  downwards,  and  hydrotropism  (response  to 
the  moist  paper)  will  direct  them  upwards.  In  this  way 
they  will  pursue  a  devious  course,  now  directed  by  one 
response  and  now  by  the  other. 

If  a  root  system  be  examined  it  will  be  found  that  when 
there  is  a  main  axis  {tap 
root)  it  is  directed 
steadily  downwards, 
while  the  branches  are 
directed  differently. 
This  indicates  that  all 
parts  of  a  root  system 
are  not  alike  in  their 
response  to  these  influ- 
ences. Several  other 
influences  are  also  con- 
cerned in  directing  soil 
roots,  and  the  path  of 
any  root  branch  is  a 
result  of  all  of  them. 
How  variable  they  are 
may  be  seen  by  the 
numerous  directions  in 
which    the    branches 

travel,  and  the  whole  root  system  preserves  the  record  of 
these  numerous  paths. 

(2)  The  pull  on  the  stem. — Another  root  property  may 
be  noted  in  connection  with  the  soil  root,  namely  the  pull 
on  the  stem.  When  a  strawberry  runner  strikes  root  at 
tip  (see  Fig.  47),  the  roots,  after  they  obtain  anchorage  in 
the  soil,  pull  the  tip  a  little  beneath  the  surface,  as  if  they 
had  gripped  the  soil  and  then  slightly  contracted.  The 
same  thing    may  be   observed    in  the   process  known  as 


Fig.  88.  A  section  through  the  stem  of  a  water- 
wort  (Matine),  showing  the  remarkably  large 
and  regularly  arranged  air  passages  for  root 
aeration.  The  single  reduced  vascular  bundle 
is  central  and  connected  with  the  small  cor- 
tex by  thin  plates  of  cells  which  radiate  like 
the  spokes  of  a  wheel. — After  Schenck. 


94 


l'LANT    RELATIONS. 


"layering,"  by  which  a  stem,  as  a  bramble,  is  bent  down 
and  covered  with  soil.  The  covered  joints  strike  root,  and 
the  pulling  follows  (see  Fig.  86).  A  very  plain  illustration 
of  this  pulling  by  roots  can  be  obtained  from  many  tuberous 
plants.  Tubers,  bulbs,  rootstocks,  etc.,  are  underground 
structures  which  have  been  observed  to  bury  themselves 
deeper  and  deeper  in  the  soil.    This  is  effected  by  the  young 


Fig.  89.    Section  through  the  leaf  of  a  quillwort  (Isoetes),  showing  the  four  large  air 
chambers  («).  the  central  vascular  region  (b),  and  the  very  poorly  developed  cortex. 

roots  which  they  continue  to  put  forth.  These  roots  grip 
the  soil,  then  contract,  and  the  tuber  is  pulled  a  little  deeper. 
The  compact  tuber  known  as  the  Indian  turnip  ("  Jack-in- 
the-pulpit ")  has  been  found  to  bury  itself  very  deeply  and 
rapidly,  and  this  may  be  observed  by  transplanting  a  young 
and  vigorous  tuber  into  a  pot  of  loose  soil. 

(3)  Soil  dangers. — In  this  connection  certain  soil  dan- 
gers and  the  response  of  the  roots  should  be  noted.  The 
soil  may  become  poor  in  water  or  poor  in  certain  essential 
materials,  and  this  results  in  an  extension  of  the  root  sys- 


KOOTS. 


95 


tern,  as  if  seeking  for  water  and  the  essential  materials. 
Sometimes  the  root  system  becomes  remarkably  extensive, 
visiting  a  large  amount  of  soil  in  order  to  procure  the 
necessary  supplies.  Sometimes  the  soil  is  poor  in  heat,  and 
root  activity  is  interfered  with.  In  such  cases  it  is  very 
common  to  find  the  leaves 
massed  against  the  soil,  thus 
slightly  checking  the  loss  of 
heat. 

Most  soil  roots  also  need  free 
air,  and  when  water  covers  the 
soil  the  supply  is  cut  off.  In 
many  cases  there  is  some  Avay 
by  which  a  supply  of  free  air 
may  be  brought  down  into  the 
roots  from  the  parts  above 
water  ;  sometimes  by  large  air 
passages  in  leaves  and  stems 
(see  Figs.  87,  88,  89,  90)  ;  some- 
times by  developing  special  root 
structures  which  rise  above  the 
water  level,  as  prominently 
shown  by  the  cypress  in  the 
development  of  knees.  These 
knees  are  outgrowths  from  roots 
beneath  the  water  of  the  cypress 
swamp,  and  rise  above  the  water  level,  thus  reaching  the 
air  and  aerating  the  root  system  (see  Fig.  91).  It  has  been 
shown  that  if  the  water  rises  so  high  as  to  flood  the  knees 
for  any  length  of  time  the  trees  will  die,  but  it  does  not 
follow  that  this  is  the  chief  reason  for  their  development. 

65.  Water  roots. — A  very  different  type  of  root  is  devel- 
oped if  it  is  exposed  to  free  water,  without  any  soil  relation. 
If  a  stem  is  floating,  clusters  of  whitish  thread-like  roots 
usually  put  out  from  it  and  dangle  in  the  water.  If  the  water 
level  sinks  so  as  to  bring  the  tips  of  these  roots  to  the  mucky 


Fig.  90.  Longitudinal  section 
through  a  young  quillwort  leaf, 
showing  that  the  four  air  cham- 
bers shown  in  Fig.  80  are  not  con- 
tinuous passages,  but  that  there 
are  four  vertical  rows  of  promi- 
nent chambers.  The  plates  of 
cells  separating  the  chambers  in 
a  vertical  row  very  soon  become 
dead  and  full  of  air.  In  addition 
to  the  work  of  aeration  these  air 
chambers  are  very  serviceable  in 
enabling  the  leaves  to  float  when 
they  break  off  and  carry  the  com- 
paratively heavy  spore  cases. 


a  5 

S  s 


a  m 


a   m  a 

o    s   a 

.9   2   ft 


ROOTS. 


97 


imssH 


Pia.  92. 


A  tropical  aroid  (Anthuriicm),  Bhowing  its  large  leaves,  and  bunches  of 
aerial  roots. 


soil  they  usually  do  not  penetrate  or  enter  into  any  soil  re- 
lation. Such  pure  water  roots  may  be  found  dangling  from 
the  under  surface  of  the  common  duck  weeds,  which  often 
cover  the  surface  of  stagnant  water  with  their  minute, 
green,  disk -like  bodies. 


98 


PLANT  RELATIONS. 


Plants  which  ordinarily  develop  soil  roots,  if  brought 
into  proper  water  relations,  may  develop  water  roots.  For 
instance,  willows  or  other  stream  bank  plants  may  be  so 
close  to  the  water  that  some  of  the  root  system  enters  it. 
In  such  cases  the  numerous  clustered  roots  show  their  water 


Fig.  93.    An  orchid,  showing  aerial  roots. 

character.  Sometimes  root  systems  developing  in  the  soil 
may  enter  tile  drains,  when  water  roots  will  develop  in  such 
clusters  as  to  choke  the  drain.  The  same  bunching  of  water 
roots  may  be  noticed  when  a  hyacinth  bulb  is  grown  in  a 
vessel  of  water. 

66.  Air  roots. — In  certain  parts  of  the  tropics  the  air  is 
so  moist  that  it  is  possible  for  some  plants  to  obtain  suffi- 


KOOTS. 


99 


cient  moisture  from  this  source,  without  any  soil-relation 
or  water-relation.  Among  these  plants  the  orchids  are 
most  notable,  and  they  may  be  observed  in  almost  any 
greenhouse.  Clinging  to  the  trunks  of  trees,  usually  imi- 
tated in  the  greenhouse  by  nests  of  sticks,  they  send  out 
long  roots  which  dangle  in  the  moist  air  (see  Figs.  93,  94). 
It  is  necessary  to  have  some  special  absorbing  arrange- 
ment, and  in  the  orchids  this 
is  usually  provided  by  the  de- 
velopment of  a  sponge-like 
tissue  about  the  root  known 
as  the  velamen,  which  greed- 
ily absorbs  the  dew  or  water 
trickling  down  the  plant.  See 
also  Figs.  92,  95,  96,  97. 

67.  Clinging  roots. — These 
roots  are  developed  to  fasten 
the  plant  body  to  some  sup- 
port, and  do  no  work  of  ab- 
sorption (see  Fig.  98).  Very 
common  illustrations  may  be 
obtained  from  the  ivies,  the 
trumpet  creeper,  etc.  These 
roots  cling  to  various  supports, 
stone  walls,  tree  trunks,  etc., 
by  sending  minute  tendril- 
like branches  into  the  crevices.  The  sea-weeds  (alga?) 
develop  grasping  structures  extensively,  a  large  majority 
of  them  being  anchored  to  rocks  or  to  some  rigid  support 
beneath  the  water,  and  their  bodies  floating  free.  The 
root-like  processes  by  which  this  anchorage  is  secured  are 
very  prominent  in  many  of  the  common  marine  sea-weeds 
(see  Fig.  157). 

68.  Prop  roots. — Some  roots  are  developed  to  prop 
stems  or  wide-spreading  branches.  In  swampy  ground,  or 
in  tropical  forests,  it  is  very  common  to  find  the  base  of 

8 


Fig.  94.      An  orchid,   showing 
roots  and  thick  leaves. 


aerial 


Fig.  a5. 


Astaghorn  fern  (Platycerlum),  an  aerial  plant  of  the  tropics.    About  it  is  a 
vine,  which  shows  the  leaves  adjusted  to  the  lighted  side. 


Fig.  96.    Selaginella,  showing  dangling  rhizophores  and  finely  divided  leaves. 


Fig.  97.    Live  oaks,  in  the  Gulf  States,  upon  which  are  growing  masses  of  long  moss 
or  black  moss  ( Tlllandsia),  a  common  aerial  plant. 


Fig.  98.  A  tropical  forest,  showing  the  cord-like  holdfasts  developed  by  an  epi- 
phyte, Which  pass  around  the  tree  trunks  like  tightly  bound  ropes.  After 
Kekneh. 


ROOTS 


103 


tree  trunks  buttressed  by  such  roots  which  extend  out  over 
and  beneath  the  surface,  and  divide  the  area  about  the  tree 
into  a  series  of  irregular  chambers  (see  Fig.  100).      Some- 


Pig.  99.     A  screw-pine  (Pandanus),  from  the    Indian  Ocean   region,  showing  the 
prominent  prop  roots  put  out  near  the  base. 

times  a  stem,  either  inclined  or  with  a  poorly  developed 
primary  root  system,  puts  out  prop  roots  which  support 
it,  as  in  the  screw-pine   (see  Tig.  99).     A  notable  case  is 


r;f    ,;  ••■  ■     ■■■■- *t-*\. 

106 


PLANT  RELATIONS. 


that  of  the  banyan  tree,  whose  wide-spreading  branches 
are  supported  by  prop  roots,  which  are  sometimes  very 
numerous  (see  Fig.  101).     The  immense  banyans   usually 

illustrated  are 
especially  culti- 
vated as  sacred 
trees,  the  prop 
roots  being  as- 
sisted in  pene- 
trating the  soil. 
There  is  record 
of  such  a  tree  in 
Ceylon  with  350 
large  and  3,000 
small  prop  roots, 
able  to  cover  a 
village  of  100 
huts. 

69.  Parasites. 
— Besides  the 
roots  mentioned 
above,  certain 
plants  develop 
root-like  p  r  o- 
cesses  which  re- 
late themtohosts. 
A  host  is  a  liv- 
ing plant  or 
animal  upon 
which  some 
other  plant  or 
animal  is  living 
as  a  parasite. 
The  parasite  gets  its  supplies  from  the  host,  and  must  be 
related  to  it  properly.  If  the  parasite  grows  upon  the 
surface  of  its  host,  it  must  penetrate  the  body  to  obtain 


Fig.  102.  A  dodder  plant  parasitic  on  a  willow  twig.  The 
leafless  dodder  twines  about  the  willow,  and  sends  out 
sucking  processes  which  penetrate  and  absorb. — After 
Strasburger. 


ROOTS. 


107 


food  supplies. 
Therefore,  pro- 
cesses are  devel- 
oped which  pene- 
trate and  absorb. 
The  mistletoe  and 
dodder  are  seed- 
plants  which  have 
this  habit,  and 
both  have  such 
processes  (see  Figs. 
102,  103).  This 
habit  is  much  more 
extensively  devel- 
oped, however,  in 
a  low  group  of 
plants  known  as 
the  fungi.  Many 
of  these  parasitic 
fungi  live  upon 
plants  and  animals, 
common  illustrations  being  the  mildews  of  lilac  leaves  and 
many  other  plants,  the  rust  of  wheat,  the  smut  of  corn,  etc. 

70.  Root  structure. 
— In  the  lowest  groups 
of  plants  (alga3,  fungi, 
and  moss-plants)  true 
roots  are  not  formed, 
but  very  simple  struc- 
tures, generally  hair- 
like (see  Fig.  104).  In 
fern-plants  and  seed- 
plants,  however,  the 
root  is  a  complex 
structure,  so  different 
from  the  root-like  pro- 


Fig.  103.  A  section  showing  the  living  connection 
between  dodder  and  a  golden  rod  upon  which  it  is 
growing.  The  penetrating  and  absorbing  organ  (h) 
has  passed  through  the  cortex  (c),  the  vascular 
zone  (6),  and  is  disorganizing  the  pith  (p). 


Pig.  104.  Section  through  the  thallus  of  a  liver- 
wort (Marchantla),  showing  the  hair-like  pro- 
cesses (rhizoids)  which  come  from  the  under 
surface  and  act  as  roots  in  gripping  and  ab- 
sorbing. In  the  epidermis  of  the  upper  surface 
a  chimney-like  opening  is  seen,  leading  into 
a  chamber  containing  cells  with  chloroplasts. 


108 


PLANT   RELATIONS. 


cesses  of  the  lower  groups  that  it  is  regarded  as  the  only 
true  root.     It  is  quite  uniform  in  structure,  consisting  of 

a  tough  and  fibrous 
central  axis  surround- 
ed by  a  spongy  region 
(Fig.  105).  The 
tough  axis  is  made 
up  mostly  of  ves- 
sels, so  called  be- 
cause they  conduct 
material,  and  is  called 
the  vascular  axis. 
The  outer  more 
spongy  region  is  the 
cortex,  which  covers 
the  vascular  axis  like 
a  thick  skin. 

One  of  the  pecu- 
liarities of  the  root  is 
that  the  branches 
come  from  the  vascu- 
lar axis  and  burrow 
through  the  cortex, 
so  that  when  the  lat- 
ter is  peeled  off  the 
branches  are  left  at- 
tached to  the  axis, 
and  the  cortex  shows 
the  holes  through 
which  they  passed. 

Another  pecu- 
liarity of  the  root  is 
that  it  elongates  only  by  growth  at  the  tip,  and  in  the  soil 
this  delicate  growing  tip  is  protected  by  a  little  cap  of  cells, 
known  as  the  root-cap. 


Pig.  105.  A  longitudinal  section  through  the  root 
tip  of  spiderwort,  showing  the  central  vascular 
axis  (pi),  surrounded  by  the  cortex  (p),  outside 
of  the  cortex  the  epidermis  (e)  which  disappears 
in  the  older  parts  of  the  root,  and  the  promi- 
nent root-cap  (c). 


CHAPTER  VI. 

REPRODUCTIVE  ORGANS. 


It  will  be  remembered  that  nutrition  and  reproduction 
are  the  two  great  functions  of  plants.  In  discussing 
foliage  leaves,  stems,  and  roots,  they  were  used  as  illustra- 
tions of  nutritive  organs,  so  far  as  their  external  relations 
are  concerned.  We  shall  now 
briefly  study  the  reproductive 
organs  from  the  same  point 
of  view,  not  describing  the 
processes  of  reproduction,  but 
some  of  the  external  relations. 
—  71.  Vegetative  multiplica- 
tion.— Among  the  very  lowest 
plants  no  special  organs  of 
reproduction  are  developed, 
but  most  plants  have  them. 
There  is  a  kind  of  reproduc- 
tion by  which  a  portion  of 
the  parent  body  is  set  apart  to 
produce  a  new  plant,  as  when 
a  strawberry  runner  produces 
a  new  strawberry  plant,  or 
when  a  willow  twig  or  a  grape 
cutting  is  planted  and  produces  new  plants,  or  when  a  potato 
tuber  (a  subterranean  stem)  produces  new  potato  plants,  or 
when  pieces  of  Begonia  leaves  are  used  to  start  new  Begonias. 
This  is  known  as  vegetative  multiplication,  a  kind  of  repro- 
duction which  does  not  use  special  reproductive  organs. 


Fig.  106.  A  group  of  spores :  A, 
spores  from  a  common  mold  (a 
fungus),  which  are  so  minute  and 
light  that  they  are  carried  about  by 
the  air  ;  B,  two  spores  from  a  com- 
mon alga  {Ulothrix),  which  can 
swim  by  means  of  the  hair-like 
processes;  C,  the  conspicuous  dotted 
cell  is  a  spore  developed  by  a  com- 
mon mildew  (a  fungus),  which  is 
carried  about  by  currents  of  air. 


110 


PLANT   RELATIONS. 


72.  Spore  reproduction. — Besides  vegetative  multiplica- 
tion most  plants  develop  special  reproductive  bodies, 
known  as  spores,  and  this  kind  of  reproduction  is  known 
spore  reproduction.  These  spores  are  very  simple 
bodies,  but  have  the  power  of  producing  new  individuals. 
There  are  two  great  groups  of  spores,  differing  from  each 
other  not  at  all  in  their  powers,  but  in  the  method  of  their 
production   by  the   parent  plant.     One  kind  of  spore  is 

produced  by  dividing 
certain  organs  of  the 
parent ;  in  the  other 
case  two  special  bodies 
of  the  parent  blend 
together  to  form  the 
spore.  Although  they 
are  both  spores,  for 
convenience  we  may 
call  the  first  kind 
spores  (see  Figs.  106, 
109),  and  the  second 
kind  eggs  (see  Fig. 
107).*  The  two  special 
bodies  which  blend  to- 
gether to  form  an  egg 
are  called  gamei&s  (see 
Figs.  107,  108,  109).  These  terms  are  necessary  to  any 
discussion  of  the  external  relations.  Most  plants  develop 
both  spores  and  eggs,  but  they  are  not  always  equally  con- 
spicuous. Among  the  algae,  both  spores  and  eggs  are  prom- 
inent ;  among  certain  fungi  the  same  is  true,  but  many 
fungi  are  not  known  to  produce  eggs  ;  among  moss-plants 
the  spores  are  prominent  and  abundant,  but  the  egg  is 
concealed  and  not  generally  noticed.     What  has  been  said 

*  It  is  recognized  that  this  spore  is  really  a  fertilized  egg,  but  in 
the  absence  of  any  accurate  simple  word,  the  term  egg  is  used  for  con- 
venience. • 


Fig.  107.  Fragments  of  a  common  alga  (Spi- 
rogyra).  Portions  of  two  threads  are  shown, 
which  have  been  joined  together  by  the  grow- 
ing of  connecting  tubes.  In  the  upper  thread 
four  cells  are  shown,  three  of  which  contain 
eggs  (s),  while  the  cell  marked  g,  and  its  mate 
of  the  other  thread  each  contain  a  gamete, 
the  lower  one  of  which  will  pass  through  the 
tube,  blend  with  the  upper  one,  and  form 
another  egg. 


REPRODUCTIVE   ORGANS. 


Ill 


of  the  moss-plants  is  still  more  true 
of  the  fern-plants ;  while  among 
the  seed-plants  certain  spores  {pol- 
len grains)  are  conspicuous  (see 
Fig.  110),  but  the  eggs  can  be  ob- 
served only  by  special  manipulation 
in  the  laboratory.  Seeds  are  neither 
spores  nor  eggs,  but  peculiar  repro- 
ductive bodies  which  the  hidden 
egg  has  helped  to  produce. 

73.  Germination.  —  Spores  and 
eggs  are  expected  to  germinate ; 
that  is,  to  begin  the  development 
of  a  new  plant.  This  germination 
needs  certain  external  conditions, 
prominent  among  which  are  defi- 
nite amounts  of  heat,  moisture, 
and  oxygen,  and  sometimes  light. 
Conditions  of  germination  may  be 
observed  most  easily  in  connection 
with  seeds.  It  must  be  understood, 
however,  that  what  is  called  the 
germination  of  seeds  is  something 


Fig.  108.  A  portion  of  the 
body  of  a  common  alga 
(QSclogonhtm),  showing 
gametes  of  very  unequal  size 
and  activity ;  a  very  large 
one  (o)  is  lying  in  a  globular 
cell,  and  a  very  small  one  is 
entering  the  cell,  another 
similar  one  (s)  being  just 
outside.  The  two  small 
gametes  have  hair-like  pro- 
cesses and  can  swim  freely. 
The  small  and  large  gam- 
etes unite  and  form  an  egg. 


Fig.  109.  A  group  of  swim- 
ming cells  :  -4,  a  spore  of 
(Edogonium  (an  alga) ; 
B,  spores  of  Ulothrix  (an 
alga) ;  C,  a  gamete  of 
Equiselum  (horse-tail  or 
scouring  rush). 


very  different  from  the  germination 
of  spores  and  eggs.  In  the  latter 
cases,  germination  includes  the  very 
beginnings  of  the  young  plant.  In 
the  case  of  a  seed,  germination  begun 
by  an  egg  has  been  checked,  and 
seed  germination  is  its  renewal.  In 
other  words,  an  egg  has  germinated 
and  produced  a  young  plant  called 
the  "embryo,"  and  the  germination 
of  the  seed  simply  consists  in  the 
continued  growth  and  the  escape  of 
this  embryo. 


112 


PLANT   RELATIONS. 


Fig.  110.  A  pollen  grain  (spore)  from  the 
pine,  which  develops  wings  (w)  to  assis* 
in  its  transportation  by  currents  of  air. 


It  is  evident  that  for 
the  germination  of  seeds 
light  is  not  an  essential 
condition,  for  they  may 
germinate  in  the  light  or 
in  the  dark  ;  but  the  need 
of  heat,  moisture,  and 
oxygen  is  very  apparent. 
The  amount  of  heat  re- 
quired for  germination 
varies  widely  with  different 
seeds,  some  germinating 
at  much  lower  tempera- 
tures than  others.     Every 

kind  of  seed,  or  spore,  or  egg  has  a  special  temperature 

range,  below  which  and  above  which 

it  caunot  germinate.     The  two  limits 

of    the    range    may    be    called    the 

lowest   and   highest  points,   but  be- 
tween the  two  there  is  a  best  point 

of  temperature  for  germination.    The 

same  general  fact  is  true  in  reference 

to  the  moisture  supply. 

74.  Dispersal  of  reproductive  bodies. 

— Among  the  most  striking  external 

relations,    however,    are    those    con- 
nected with  the  dispersal  of  spores, 

gametes,  and  seeds.     Spores  and 

seeds  must  be  carried  away  from  the 

parent    plant,    and    separated    from 

each    other,    out    of    the    reach    of 

rivalry   for   nutritive    material ;   and 

gametes    must    come    together    and 

blend  to  form  the  eggs.    Conspicuous        (Epiiobium)  opening  and 

among  the  means  of  transfer  are  the        exposing  its  plumed  seeds 

°  which  are  transported  by 

following.  the  wind.-After  Bkal. 


REPRODUCTIVE   ORGANS. 


lib 


75.  Dispersal  by  locomotion. — The   common  method  of 
locomotion  is  by  means  of  movable  hairs  (cilia)  developed 
upon  the  reproductive  body,  which  propel  it  through  the 
water  (see  Fig.   109). 
Swimming  spores  are 
very   common   among 
the  algse,  and  at  least 
one    of    the    gametes 
in  algee,   moss-plants, 
and    fern-plants    has 
the    power   of    swim- 
ming   by   means    of 
cilia. 

7G.  Dispersal  by 
water.  —  It  is  very 
common  for  repro- 
ductive bodies  to  be 
transported  by  cur- 
rents of  water.  The 
spores  of  many  water 
plants  of  all  groups, 
not  constructed  for 
locomotion,  are  thus 
floated  about.  This 
method  of  transfer  is 
also  very  common 
among  seeds.  Many 
seeds  are  buoyant,  or 
become  so  after  soak- 
ing in  water,  and 
may  be  carried  to 
great   distances   by 

currents.  For  this  reason  the  plants  growing  upon  the 
banks  or  flood-plains  of  streams  may  have  come  from  a 
wide  area.  Many  seeds  can  even  endure  prolonged  soak- 
ing in  sea-water,  and  then  germinate.     Darwin  estimated 


Fig.  112.  The  upper  figure  to  the  left  is  ar  opening 
pod  of  fireweed  discharging  its  plumed  seeds. 
The  lower  figure  represents  the  seed-like  fruits 
of  Clematis  with  their  long  tail-like  plumes.— 
After  Kerner. 


114 


PLANT   RELATIONS. 


that  at  least 
fourteen  per 
cent,  of  the 
seeds  of  any 
country  can  re- 
tain their  vital- 
ity in  sea-water 
for  twenty- 
eight  days.  At 
the  ordinary 
rate  of  move- 
ment of  ocean 
currents,  this 
length  of  time 
would  permit 
such  seeds  to 
be  transported 
over  a  thou- 
sand miles, 
thus  making 
possible   a   very   great   range   in   distribution. 

77.  Dispersal  of  spores  by  air. — This  is  one  of  the  most 
common  methods  of  transport- 
ing spores  and  seeds.  In  most 
cases  spores  are  sufficiently 
small  and  light  to  be  trans- 
ported by  the  gentlest  move- 
ments of  air.  Among  the 
fungi  this  is  a  very  common 
method  of  spore  dispersal  (see 
Fig.  106),  and  it  is  extensively 
used  in  scattering  the  spores 
of  moss-plants,  fern-plants  (see 
Fig.  45),  and  seed-plants. 
Among  seed-plants  this  is  one    FlG-  114-  8eed-like  fraitf>  of  «««*> 

, ,       ,       .-IT        ,.  wltn  plumes  for  dispersal  by  air.— 

method  of  pollination,  the        After  kbbnbb. 


Fig.  118.  A  ripe  dandelion  head,  showing  the  mass  of 
plumes,  a  few  seed-like  fruits  with  their  plumes  still 
attached  to  the  receptacle,  and  two  fallen  off.—  After 
Kerner. 


REPRODUCTIVE   ORGANS 


115 


Fig.  115.    A  winged  seed  of  Bignonia— After  Strasburger. 


spores  called  pollen 
and  occasionally 
falling  upon  the 
right  spot  for 
germination. 
With  such  an 
agent  of  transfer 
the  pollen  must 
be  very  light  and 
powdery,  and 
also  very  abun- 
dant, for  it  must 
come  down  al- 
most like  rain  to  be 


grains  being  scattered  by  the  wind, 


Fig.  117. 


Winged    fruit   of 
Kerner. 
9 


Fig.  116.    Winged  fruit  of  maple.— After  Kerner. 

certain  of  reaching  the  right  places. 
Among  the  gymno- 
sperms  (pines,  hem- 
locks, etc.)  this  is  the 
exclusive  method  of 
pollination,  and  when  a 
pine  forest  is  shedding 
pollen  the  air  is  full  of 
the  spores,  which  may 
be  carried  to  a  great 
distance  before  being 
deposited.        Occasional 


Ptelea.— After 


116 


PLANT   RELATIONS. 


Fig.  118.    Winged  fruit  of 
Ailanlhus.—  After  Ker- 

NEK. 


reports  of  "showers  of  sulphur"  have 
arisen  from  an  especially  heavy  fall  of 
pollen  that  has  been  carried  far  from 
some  gymnosperm  forest.  In  the  case 
of  pines  and  their  near  relatives,  the 
pollen  spores  are  assisted  in  their  dis- 
persal through  the  air  by  developing  a 
pair  of  brpad  wings  from  the  outer 
coat  of  the  spore  (see  Fig.  110).  This 
same  method  of  pollination — that  is, 
carrying  the  pollen  spores  by  currents 
of  air — is  also  used  by  many  mono- 
cotyledons, such  as  grasses ;  and  by 
many  dicotyledons,  such  as  our  most 
common  forest  trees 
(oak,  hickory,  chest- 
nut, etc.). 

78.  Dispersal  of 
seeds  by  air. — Many 
seeds  are  carried 
about  in  various  ways 
by  currents  of  air 
without  any  special 
adaptation.  Wings 
and  plumes  of  very 
many  and  often  very 
beautiful  patterns 
are  exceedingly  com- 
mon in  connection 
with  seeds  or  seed- 
like fruits  (see  Figs. 
115,  116,  117,  118, 
119).  Wings  are  de- 
veloped by  the  fruit 
of    maples    and   of  «_,*.,.         ^  ,m„  ,   ,  ♦». 

r  Fig.  119.    Fruit  of  basewood  ( Tiha),  showing  the 

ash,  and  by  the  Seeds  peculiar  wing  formed  by  a  leaf  .—After  Kerner. 


REPRODUCTIVE   ORGANS. 


117 


Pig.  120.    A  common  tumbleweed  (Cyclolomd). 


of  pine  and  catalpa.  Plumes  and  tufts  of  hairs  are  devel- 
oped by  the  seed-like  fruits  of  dandelion,  thistle,  and  very 
many  of  their  relatives,  and  by  the  seeds  of  the  milkweed 
(see  Figs.  Ill,  112, 113,  111).  On  plains,  or  level  stretches, 
where  winds  are 
strong,  a  curious 
habit  of  seed  dis- 
persal has  been  de- 
veloped by  certain 
plants  known  as 
"  tumbleweeds  "  or 
"  field  rollers.  " 
These  plants  are 
profusely  branching 
annuals  with  a  small 

Fis.  121.    The  3-valved  fruit  of  violet  discharging 
root     system     in    a  its  eeeds.-After  Beal. 


118 


PLANT  RELATIONS. 


Fig.  122.  A  fruit  of  witch 
hazel  discharging  its 
seeds.— After  Beai. 


light  or  sandy  soil  (see  Fig.  120). 
When  the  work  of  the  season  is  over, 
and  the  absorbing  rootlets  have 
shriveled,  the  plant  is  easily  broken 
from  its  roots  by  a  gust  of  wind, 
and  is  trundled  along  the  surface  like 
a  light  wicker  ball,  the  ripe  seed  ves- 
sels, dropping  their  seeds  by  the  way. 
In  case  of  an  obstruction,  such  as  a 
fence,  great  masses  of  these  tumble- 
weeds  may  often  be  seen  lodged 
against  the  windward  side. 

79.  Discharge  of  spores. — In  many 
plants  the  distribution  of  spores  and 
seeds  is  not  provided  for  by  any  of 

the  methods  just  mentioned,  but  the  vessels  containing 

them  are  so  constructed   that  they  are  discharged  with 

more  or  less  violence  and  are  some- 
what scattered. 

Many  spore  cases,  especially  those 

of  the  lower  plants,  burst  irregularly, 

and  with  sufficient  violence  to  throw 

out  spores.     In  the  liverworts  pecu- 
liar cells,  called  elaters  or  "jumpers," 

are  formed  among  the  spores,  and 

when  the  wall  of  the  spore  case  is 

ruptured  the  elaters  are  liberated, 

and  by  their  active  motion  assist  in 

discharging  the  spores. 

In  most  of  the  true  mosses  the 

spore  case  opens  by  pushing  off  a 

lid  at   the   apex,    which   exposes  a 

delicate  fringe  of  teeth  covering  the 

mouth  of  the  urn-like  case.     These 

teeth  bend  in  and  out  of  the  open 

spore  case  as  they  become  moist  or 


Fig.  123.  A  pod  of  wild  bean 
bursting,  the  two  valves 
violently  twisting  and  dis- 
charging the  seeds.— After 
Beai.. 


REPRODUCTIVE  ORGANS. 


119 


dry,  and  are  of  considerable  service 

in  the  discharge  of  spores. 

In   the    common   ferns   a   heavy 

spring-like    ring   of    cells    encircles 

the   delicate-walled  spore   case. 

When   the   wall    becomes    dry   and 

comparatively     brittle     the     spring 

straightens  with  considerable  force, 

the  delicate  wall  is  suddenly  torn, 

and  in  the  recoil  the  spores  are  dis- 
charged (see  Fig.  45). 

Even  in  the  case  of  the  pollen- 
spores  of  seed-plants,  a  special  layer 

of  the  wall  of  the  pollen-sac  usually 

develops  as  a  spring-like  layer,  which 

assists  in  opening  widely  the  sac 
when  the  wall  be- 
gins to  yield  along 
the  line  of  break- 
ing. 

80.  Discharge  of 
seeds. — While  seeds  are  generally  carried 
away  from  the  parent  plant  by  the  agency 
of  water  currents  or  air  currents,  as  al- 
ready noted,  or  by  animals,  in  some  in- 
stances there  is  a  mechanical  discharge 
provided  for  in  the  structure  of  the  seed- 
case.  In  such  plants  as  the  witch  hazel 
and  violet,  the  walls  of  the  seed-vessel 
press  upon  the  contained  seeds,  so  that 
when  rupture  occurs  the  seeds  are  pinched 

fig.  125.  a  fruit  of     out,  as  a  moist  apple-seed  is  discharged 
begsar  ilc!L8'     by  being  pressed  between  the  thumb  and 

snowing  the  two  •'  °    r 

barbed    append-     finger  (see  Figs.  121,  122).    In  the  touch- 
ages  which   lay     me-not  a  strain  is  developed  in  the  wall 

hold  of  animals.  l 

— After  beai>-         of  the  seed-vessel,  so  that  at  rupture  it 


Fig.  124.  Fruits  of  Spanish 
needle,  showing  barbed  ap- 
pendages for  grappling. 
The  figure  to  the  left  is  one 
of  the  fruits  enlarged.— 
After  Kerner. 


120 


PLANT   RELATIONS. 


Fig.  126.  The  fruit  of  carrot,  showing 
the  grappling  appendages. — After 
Beal. 


suddenly  curls  up  and  throws  the  seeds  (see  Fig.  123).  The 
squirting  cucumber  is  so  named  because  it  becomes  very 
much  distended  with  water,  which  is  finally  forcibly  ejected 
along  with  the  mass  of  seed.    An  "  artillery  plant "  common 

in  cultivation  discharges  its 
seeds  with  considerable  vio- 
lence. ;  while  the  detonations 
resulting  from  the  explosions 
of  the  seed-vessels  of  Hura 
crepitans,  the  "monkey's  din- 
ner bell,"  are  often  remarked 
by  travelers  in  tropical 
forests. 

81.  Dispersal  of  seeds  by  animals. — Only  a  few  illustra- 
tions can  be  given  of  this  very  large  subject.  Water  birds 
are  great  carriers  of  seeds  which  are  contained  in  the  mud 
clinging  to  their  feet  and  legs.  This  mud  from  the  borders 
of  ponds  is  usually  completely  filled  with  seeds  and  spores 
of  various  plants.  One  has  no  conception  of  the  number 
until  they  are  actually  com- 
puted. The  following  ex- 
tract from  Darwin's  Origin 
of  Species  illustrates  this 
point : 

' '  I  took,  in  February,  three 
tablespoonf uls  of  mud  from  three 
different  points  beneath  water, 
on  the  edge  of  a  little  pond.  This  mud  when  dried  weighed  only  6| 
ounces  ;  I  kept  it  covered  up  in  my  study  for  six  months,  pulling  up 
and  counting  each  plant  as  it  grew  ;  the  plants  were  of  many  kinds, 
and  were  altogether  537  in  number ;  and  yet  the  viscid  mud  was  all 
contained  in  a  breakfast  cup  !  " 

Water  birds  are  generally  high  and  strong  fliers,  and  the 
seeds  and  spores  may  thus  be  transported  to  the  margins  of 
distant  ponds  or  lakes,  and  so  very  widely  dispersed. 

In  many  cases  seeds  or  fruits  develop  grappling  append- 


Fig.  127.    The  fruit  of  cocklebur,  showing 
the  grappling  appendages. —After  Beal. 


REPRODUCTIVE   ORGANS. 


121 


ages  of  various  kinds,  which  lay  hold  of  animals  brushing 
past,  and  so  the  seeds  are  dispersed.  Common  illustrations 
are  Spanish  needles,  beggar  ticks,  stick  seeds,  burdock,  etc. 
Study  Figs.  124,  125,  126,  127,  128,  129,  130. 


Fib.  128.    Fruits  with  grappling  appendages.    That  to  the  left  is  agrimony  ;  that  to 
the  right  is  Galium. — After  Kerner. 

In  still  other  cases  the  fruit  becomes  pulpy,  and  attrac- 
tive as  food  to  certain  birds  or  mammals.  Many  of  the 
seeds  (such  as  those  of  grapes)  may  be  able  to  resist  the 
attacks  of  the  digestive  fluids  and  escape  from  the  alimen- 
tary tract  in  a  condition  to  germinate.  As  if  to  attract  the 
attention  of  fruit-eating  animals,  fleshy  fruits  usually 
become  brightly  col- 
ored when  ripe,  so  that 
they  are  plainly  seen 
in  contrast  with  the 
foliage. 

82.  Dispersal  of  pol- 
len spores  by  insects. — 
The  transfer  of  pollen, 
the   name    applied    to     FlG-  129-    Fruits  with  §raPPlins  appendages. 

rr  The  figure  to  the  left  is  cocklebur  ;  that  to  the 

Certain    spores  Of   Seed-  right  is  burdock.— After  Kerner. 


122 


PLANT   RELATIONS. 


plants,  is  known  as  pollination,  and 
the  two  chief  agents  of  this  transfer 
are  currents  of  air  and  insects.  In 
§77  the  transfer  by  currents  of  air 
was  noted,  such  plants  being  known 
as  anemophilous  plants.  Such  plants 
seldom  produce  what  are  generally 
recognized  as  true  flowers.  All  those 
seed-plants  which  produce  more  or 
less  showy  flowers,  however,  are  in 
some  way  related  to  the  visits  of 
insects  to  bring  about  pollination, 
and  are  known  as  entomophilous 
plants.  This  relation  between  "In- 
sects and  flowers  is  so  important  and  so  extensive  that  it 
will  be  treated  in  a  separate  chapter. 


Fig.  130  A  head  of  fruits  of 
burdock,  showing  the 
grappling  appendages. — 
After  Beal. 


CHAPTEE  VII. 

FLOWERS  AND  INSECTS. 

83.  Insects  as  agents  of  pollination. — The  use  of  insects 
as  agents  of  pollen  transfer  is  very  extensive,  and  is  the  pre- 
vailing method  of  pollination  among  monocotyledons  and 
dicotyledons.  All  ordinary  flowers,  as  usually  recognized, 
are  related  in  some  way  to  pollination  by  insects,  but  it 
must  not  be  supposed  that  they  are  always  successful  in 
securing  it.  This  mutually  helpful  relation  between  flow- 
ers and  insects  is  a  very  wonderful  one,  and  in  some  cases 
it  has  become  so  intimate  that  they  cannot  exist  without 
each  other.  Flowers  have  been  modified  in  every  way  to  be 
adapted  to  insect  visits,  and  insects  have  been  variously 
adapted  to  flowers. 

84.  Self-pollination  and  cross- pollination. — The  advantage 
of  this  relation  to  the  flower  is  to  secure  pollination.  The 
pollen  may  be  transferred  to  the  carpel  of  its  own  flower, 
or  to  the  carpel  of  some  other  flower.  The  former  is  known 
as  self-pollination,  the  latter  as  cross-pollination.  In  the 
case  of  cross-pollination  the  two  flowers  concerned  may  be 
upon  the  same  plant,  or  upon  different  plants,  which  may 
be  quite  distant  from  one  another.  It  would  seem  that 
cross-pollination  is  the  preferred  method,  as  flowers  are  so 
commonly  arranged  to  secure  it. 

85.  Advantage  to  insects. — The  advantage  of  this  relation 
to  the  insect  is  to  secure  food.  This  the  flower  provides 
either  in  the  form  of  nectar  or  pollen  ;  and  insects  visiting 
flowers  may  be  divided  roughly  into  the  two  groups  of 
nectar-feeding  insects,  represented  by  butterflies  and  moths, 


124  PLANT  RELATIONS. 

and  pollen-feeding  insects,  represented  by  the  numerous 
bees  and  wasps.  When  pollen  is  provided  as  food,  the 
amount  of  it  is  far  in  excess  of  the  needs  of  pollination. 
The  presence  of  these  supplies  of  food  is  made  known  to 
the  insect  by  the  display  of  color  in  connection  wUh  the 
flowers,  by  odor,  or  by  form.  It  should  be  said  that  the 
attraction  of  insects  by  color  has  been  doubted  recently,  as 
certain  experiments  have  suggested  that  some  of  the  com- 
mon flower-visiting  insects  are  color-blind,  but  remarkably 
keen-scented.  However  this  may  be  for  some  insects,  it 
seems  to  be  sufficiently  established  that  many  insects  rec- 
ognize their  feeding  ground  by  the  display  of  color. 

86.  Suitable  and  unsuitable  insects. — It  is  evident  that 
all  insects  desiring  nectar  or  pollen  for  food  are  not  suit- 
able for  the  work  of  pollination.  For  instance,  the  ordi- 
nary ants  are  fond  of  such  food,  but  as  they  walk  from  plant 
to  plant  the  pollen  dusted  upon  them  is  in  great  danger  of 
being  brushed  off  and  lost.  The  most  favorable  insect  is 
the  flying  one,  that  can  pass  from  flower  to  flower  through 
the  air.  It  will  be  seen,  therefore,  that  the  flower  must  not 
only  secure  the  visits  of  suitable  insects,  but  must  guard 
against  the  depredations  of  unsuitable  ones. 

87.  Danger  of  self-pollination. — There  is  still  another 
problem  which  insect-pollinating  flowers  must  solve.  If 
cross-pollination  is  more  advantageous  to  the  plant  than 
self-pollination,  the  latter  should  be  prevented  so  far  as 
possible.  As  the  stamens  and  carpels  are  usually  close  to- 
gether in  the  same  flower,  the  danger  of  self-pollination  is 
constantly  present  in  many  flowers.  In  those  plants  which 
have  stamen-producing  flowers  upon  one  plant  and  carpel- 
producing  flowers  upon  another,  there  is  no  such  danger. 

88.  Problems  of  pollination. — In  most  insect-pollinating 
flowers,  therefore,  there  are  three  problems  :  (1)  to  prevent 
self-pollination,  (2)  to  secure  the  visits  of  suitable  insects, 
and  (3)  to  ward  off  the  visits  of  unsuitable  insects.  It 
must  not  be  supposed  that  flowers  are  uniformly  successful 


FLOWERS   AND   INSECTS. 


125 


in  solving  these  problems.     They  often  fail,  but  succeed 
often  enough  to  make  the  effort  worth  while. 

89.  Preventing  self-pollination. — It  is  evident  that  this 
danger  arises  only  in  those  flowers  in  which  the  stamens 
and  carpels  are  associ 
ated,   bat  their  separa-  ^.  '     ^ 

tion  in  different  flowers 
may  be  considered  as 
one  method  of  prevent- 
ing self-pollination.  In 
order  to  understand  the 
various  arrangements  to 
be  considered,  it  is  nec- 
essary to  explain  that 
the  carpel  does  not  re- 
ceive the  pollen  indif- 
ferently over  its  whole 
surface.  There  is  one 
definite  region  organ- 
ized, known  as  the 
stigma,  upon  which  the 
pollen  must  be  deposited 
if  it  is  to  do  its  work. 
Usually  this  is  at  the 
most  projecting  point 
of  the  carpel,  very  often 
at  the  end  of  a  stalk- 
like prolongation  from 
the  ovary  (the  bulbous 
part  of  the  carpel), 
known  as  the  style; 
sometimes  it  may  run  down  one  side  of  the  style.  When 
the  stigma  is  ready  to  receive  pollen  it  has  upon  it  a 
sweetish,  sticky  fluid,  which  holds  and  feeds  the  pollen. 
In  this  condition  the  stigma  is  said  to  be  mature ;  and  the 
pollen  is  mature  when  it  is  being  shed,  that  is,  ready  to  fall 


Fig.  131.  Parts  of  the  flower  of  rose  acacia 
(Robiniahispidd).  In  1  the  keel  is  shown  pro- 
jecting from  the  hairy  calyx,  the  other  more 
showy  parts  of  the  corolla  having  been  re- 
moved. Within  the  keel  are  the  stamens 
and  the  carpel,  as  seen  in  3.  The  keel  forms 
the  natural  landing  place  of  a  visiting  bee, 
whose  weight  depresses  the  keel  and  causes 
the  tip  of  the  style  to  protrude,  as  shown  in 
2.  This  style  tip  bears  pollen  upon  it, 
caught  among  the  hairs,  seen  in  3,  and  as  it 
strikes  the  body  of  the  bee  some  pollen  is 
brushed  ofiE.  If  the  bee  has  previously  visited 
another  flower  and  received  some  pollen,  it 
will  be  seen  that  the  stigma,  at  the  very  tip 
of  the  style,  striking  the  body  first,  will  very 
probably  receive  some  of  it.  The  nectar  pit 
is  shown  in  3,  at  the  base  of  the  uppermost 
stamen.— After  Gray. 


126 


PLANT   RELATIONS. 


out  of  the  pollen-sacs  or  to  be  removed  from  them.  The 
devices  used  by  flowers  containing  both  stamens  and  carpels 
to  prevent  self-pollination  are  very  numerous,  but  most 
of  them  may  be  included  under  the  three  following  heads  : 
(1)  Position. — In  these  cases  the 
pollen  and  stigma  are  ready  at  the  same 
time,  but  their  position  in  reference  to 
each  other,  or  in  reference  to  some  con- 
formation of  the  flower,  makes  it  un- 
likely that  the  pollen  will  fall  upon  the 
stigma.  The  stigma  may  be  placed 
above  or  beyond  the  pollen  sacs,  or  the 
two  may  be  separated  by  some  mechan- 
ical obstruction,  resulting  in  much  of 
the  irregularity  of  flowers. 

In  the  flowers  of  the  rose  acacia  and 
its  relatives,  the  several  stamens  and 
the  single  carpel  are  in  a  cluster,  en- 
closed in  the  keel  of  the  flower.  The 
stigma  is  at  the  summit  of  the  style, 
and  projects  somewhat  beyond  the 
pollen-sacs  shedding  pollen.  Also  there 
is  often  a  rosette  of  hairs,  or  bristles, 
just  beneath  the  stigma,  which  acts  as 
a  barrier  to  the  pollen  (see  Fig.  131). 

In  the  iris,  or  common  flag,  each 
stamen  is  in  a  sort  of  pocket  between 
the  petal  and  the  petal-like  style,  while 
the  stigmatic  surface  is  on  the  top  of  a 
flap,  or  shelf,  which  the  style  sends  out 
as  a  roof  to  the  pocket.  With  such  an 
arrangement,  it  would  seem  impossible 
for  the  pollen  to  reach  the  stigma  un- 
aided (see  Fig.  132). 

In  the  orchids,  remarkable  for  their 
strange  and  beautiful  flowers,  there  are 


Fig.  132.  A  portion  of 
the  flower  of  an  iris, 
or  flag.  The  single 
stamen  shown  is 
standing  between  the 
petal  to  the  right  and 
the  petal-like  style  to 
the  left.  Near  the 
top  of  this  style  the 
stigmatic  shelf  is 
seen  extending  to  the 
right,  which  must 
receive  the  pollen 
upon  its  upper  sur- 
face. The  nectar 
pit  is  at  the  junc- 
tion of  the  petal  and 
stamen.  While  ob- 
taining the  nectar  the 
insect  brushes  the 
pollen-bearing  part 
of  the  stamen,  and 
pollen  is  lodged  upon 
its  body.  In  visiting 
the  next  flower  and 
entering  the  stamen 
chamber  the  stig- 
matic shelf  is  apt  to 
be  brushed. — After 
Gray. 


FLOWERS   AND   INSECTS. 


127 


usually  two  pollen-sacs,  and  stretched  between  them  is  the 
stigmatic  surface.  In  this  case,  however,  the  pollen  grains 
are  not  dry  and  powdery,  but  cling  together  in  a  mass,  and 
cannot  escape  from  the  sac  without  being  pulled  out  (see 
Fig.  133).  The  same  sort  of  pollen  is  developed  by  the 
milkweeds. 

(2)  Consecutive  maturity. — In  these  cases  the  pollen  and 


Fig.  133.  A  flower  of  an  orchid  (Habenaria).  At  1  the  complete  flower  is  shown, 
with  three  sepals  behind,  and  three  petals  in  front,  the  lowest  one  of  which  has 
developed  a  long  strap-shaped  portion,  and  a  still  longer  spur  portion,  the  opening 
to  which  is  seen  at  the  base  of  the  strap.  At  the  bottom  of  this  long  spur  is  the 
nectar,  which  is  reached  by  the  long  proboscis  of  a  moth.  The  two  pollen  sacs  of 
the  single  stamen  are  seen  in  the  centre  of  the  flower,  diverging  downwards,  and 
between  them  stretches  the  stigma  surface.  The  relation  between  pollen  sacs  and 
stigma  surface  is  more  clearly  shown  in  2.  Within  each  pollen  sac  is  a  mass  of 
sticky  pollen,  ending  below  in  a  sticky  disk,  which  may  be  seen  in  1  and  2.  When 
the  moth  thrusts  his  proboscis  into  the  nectar  tube,  his  head  is  against  the  stig- 
matic surface  and  also  against  the  disks.  When  he  removes  his  head  the  disks 
stick  fast  and  the  pollen  masses  are  dragged  out.  In  3  a  pollen  mass  (a)  is 
shown  sticking  to  each  eye  of  a  moth.  Upon  visiting  another  flower  these  pollen 
masses  are  thrust  against  the  stigmatic  surface  and  pollination  is  effected.— After 

GBA7. 


128 


PLANT  RELATIONS. 


stigma  of  the  same  flower  are  not  mature  at  the  same  time. 
It  is  evident  that  this  is  a  very  effective  method  of  prevent- 
ing self-pollination.  When  the  pollen  is  being  shed  the 
stigma  is  not  ready  to  receive,  or  when  the  stigma  is  ready 
to  receive  the  pollen  is  not  ready  to  be  shed.  In  some 
cases  the  pollen  is  ready  first,  in  other  cases  the  stigma, 
the  former  condition  being  called  prota?idry,  the  latter 
protogyny.     This  is  a  very  common  method  of  preventing 

self-pollination,  and  is 
usually  not  associated  with 
irregularity. 

The  ordinary  figwort  may 
be  taken  as  an  example  of 
protogyny.  When  the  flow- 
ers first  open,  the  style,  bear- 
ing the  stigma  at  its  tip,  is 
found  protruding  from  the 
urn -like  flower,  while  the 
four  stamens  are  curved 
down  into  the  tube,  and  are 
not  ready  to  shed  their  pollen. 
At  some  later  time  the  style 
bearing  the  stigma  wilts, 
and  the  stamens  straighten 
up  and  protrude  from  the  tube.  In  this  way,  first  the 
receptive  stigma,  and  afterwards  the  shedding  pollen-sacs, 
occupy  the  same  position. 

Protandry  is  even  more  common,  and  many  illustrations 
can  be  obtained.  For  example,  the  showy  flowers  of  the 
common  firewee'd,  or  great  willow  herb,  when  first  opened 
display  their  eight  shedding  stamens  prominently,  the  style 
being  sharply  curved  downward  and  backward,  carrying 
the  four  stigma  lobes  well  out  of  the  way.  Later,  the 
stamens  bend  away,  and  the  style  straightens  up  and  ex- 
poses its  stigma  lobes,  now  receptive  (see  Fig.  134). 

(3)  Difference  in  pollen. — In  these   cases  there  are  at 


Fig.  134.  Flowers  of  fireweed  (Epi- 
lobium),  showing  protandry.  In  1  the 
stamens  are  thrust  forward,  and  the 
style  is  sharply  turned  downward  and 
backward.  In  2  the  style  is  thrust 
forward,  with  its  stigmatic  branches 
spread.  An  insect  in  passing  from  1 
to  2  will  almost  certainly  transfer  pol- 
len from  the  stamens  of  1  to  the  stig- 
mas of  2.— After  Gray. 


FLOWERS   AND   INSECTS. 


129 


least  two  forms  of  flowers,  which  differ  from  one  another 
in  the  relative  lengths  of  their  stamens  and  styles.  In  the 
accompanying  illustrations  of  Houstonia  (see  Fig.  135)  it 
is  to  be  noticed  that  in  one  flower  the  stamens  are  short 
and  included  in  the  tube,  and  the  style  is  long  and  pro- 
jecting, with  the  four  stigmas  exposed  well  above  the 
tube.  In  the 
other  flower  the 
relative  lengths 
are  exactly  re- 
versed,  the 
style  being 
short  and  in- 
cluded in  the 
tube,  and  the 
stamens  long 
and  projecting. 
It  appears  that 
the  pollen  from 
the  short  sta- 
mens is  most 
effective  upon 
the  stigmas  of 
the  short  styles, 
and  that  the 
pollen  from  the 
long  stamens  is 
most  effective 
upon  the  stig- 
mas of  the  long  styles  ;  and  as  short  stamens  and  long 
styles,  or  long  stamens  and  short  styles,  are  associated  in 
the  same  flower,  the  pollen  must  be  transferred  to  some 
other  flower  to  find  its  appropriate  stigma.  This  means 
that  there  is  a  difference  between  the  pollen  of  the  short 
stamens  and  that  of  the  long  ones. 

In  some  cases  there  are  three  forms  of  flowers,  as  in  one 


Pig.  135.  Flowers  of  Houstonia,  showing  two  forms  of 
flowers.  In  1  there  are  short  stamens  and  a  long  style  ; 
in  2  long  stamens  and  short  style.  An  insect  visiting  1 
will  receive  a  band  of  pollen  about  the  front  part  of  its 
body  ;  upon  visiting  2  this  band  will  rub  against  the 
stigmas,  and  a  fresh  pollen  band  will  be  received  upon 
the  hinder  part  of  the  body,  which,  upon  visiting  another 
flower  like  No.  1,  will  brush  against  the  stigmas.— 
After  Gray. 


130 


PLANT  RELATIONS. 


of  the  common  loosestrifes.  Each  flower  has  stamens  of 
two  lengths,  which,  with  the  style,  makes  possible  three 
combinations.  One  flower  has  short  stamens,  middle-length 
stamens,  and  long  style  ;  another  has  sh^rt  stamens,  middle- 
length  style,  and  long  stamens  ;  the  third  has  short  style, 
middle-length  stamens,  and  long  stamens.  In  these  cases 
also  the  stigmas  are  intended  to  receive  pollen  from  stamens 


Fig.  136.  Yucca  and  Pronuba.  In  the  lower  figure  to  the  right  an  opened  flower 
shows  the  pendent  ovary  with  the  stigma  region  at  its  apex.  The  upper  figure  to 
the  right  shows  the  position  of  Pronuba  when  collecting  pollen.  The  figure  to  the 
left  represents  a  cluster  of  capsules  of  Yucca,  which  shows  the  perforations  made 
by  the  larvae  of  Pronuba  in  escaping.— After  Riley  and  Trelease. 


of  their  own  length,  and  a  transfer  of  pollen  from  flower  to 
flower  is  necessary. 

90.  Self-pollination. — In  considering  these  three  general 
methods  of  preventing  self-pollination,  it  must  not  be  sup- 
posed that  self-pollination  is  never  provided  for.  It  is  pro- 
vided for  more  extensively  than  was  once  supposed.  It  is 
found  that  many  plants,  such  as  violets,  in  addition  to  the 
usual  showy,  insect-pollinated  flowers,  produce  flowers  that 
are  not  at  all  showy,  in  fact  do  not  open,  and  are  often  not 
prominently  placed.  The  fact  that  these  flowers  are  often 
closed  has   suggested  for  them  the   name  cleistogamous 


FLOWERS  AND   INSECTS.  131 

flowers.    In  these  flowers  self-pollination  is  a  necessity,  and 
is  found  to  be  very  effective  in  producing  seed. 

91.  Yucca  and  Pronuba. — There  can  be  no  doubt,  also, 
that  there  is  a  great  deal  of  self-pollination  effected  in 
flowers  adapted  for  pollination  by  insects,  and  that  the  in- 
sects themselves  are  often  responsible  for  it.  But  in  the 
remarkable  case  of  Yucca  and  Pronuba  there  is  a  definite 
arrangement  for  self-pollination  by  means  of  an  insect  (see 
Fig.  136).  Yucca  is  a  plant  of  the  southwestern  arid  regions 
of  North  America,  and  Pronuba  is  a  moth.  The  plant  and 
the  moth  are  very  dependent  upon  each  other.  The  bell- 
shaped  flowers  of  Yucca  hang  in  great  terminal  clusters,  with 
six  hanging  stamens,  and  a  central  ovary  ribbed  lengthwise, 
and  with  a  funnel-shaped  opening  at  its  apex,  which  is  the 
stigma.  The  numerous  ovules  occur  in  lines  beneath  the 
furrows.  During  the  day  the  small  female  Pronuba  rests 
quietly  within  the  flower,  but  at  dusk  becomes  very  active. 
She  travels  down  the  stamens,  and  resting  on  the  open 
pollen-sac  scoops  out  the  somewhat  sticky  pollen  with  her 
front  legs.  Holding  the  little  mass  of  pollen  she  runs  to 
the  ovary,  stands  astride  one  of  the  furrows,  and  pierc- 
ing through  the  wall  with  her  ovipositor,  deposits  an  egg 
in  an  ovule.  After  depositing  several  eggs  she  runs  to  the 
apex  of  the  ovary  and  begins  to  crowd  the  mass  of  pollen 
she  has  collected  into  the  funnel-like  stigma.  These  actions 
are  repeated  several  times,  until  many  eggs  are  deposited 
and  repeated  pollination  has  been  effected.  As  a  result  of 
all  this  the  flower  is  pollinated,  and  seeds  are  formed  which 
develop  abundant  nourishment  for  the  moth  larvae,  which 
become  mature  and  bore  their  way  out  through  the  wall  of 
the  capsule  (Fig.  136). 

92.  Securing  cross-pollination. — In  very  many  ways  flow- 
ers are  adapted  to  the  visits  of  suitable  insects.  In  ob- 
taining nectar  or  pollen  as  food,  the  visiting  insect  receives 
pollen  on  some  part  of  its  body  which  will  be  likely  to 
come  in  contact  with  the  stigma  of  the  next  flower  visited. 

10 


Fig   137.    A  clump  of  lady-slippers  (Cypripedium),  showing  the  habit  of  the  plant 
and  the  general  structure  of  the  flower.— After  Gibson. 


FLOWERS   AND   INSECTS. 


133 


Illustrations  of  this  process  may  be  taken  from  the  flowers 
already  described  in  connection  with  the  prevention  of 
self-pollination. 

In  the  flowers  of  the  pea  family,  such  as  the  rose  acacia 
(see  Fig.  131),  it  will  be 
noticed  that  the  stamens 
and  pistil  are  concealed 
within  the  keel,  which 
forms  the  natural  land- 
ing place  for  the  bees 
which  are  used  in  pol- 
lination. This  keel  is 
so  inserted  that  the 
weight  of  the  insect  de- 
presses it,  and  the  tip 
of  the  style  comes  in 
contact  with  its  body. 
Not  only  does  the 
stigma  strike  the  body, 
but  by  the  glancing 
blow  the  surface  of  the 
style  is  rubbed  against 
the  insect,  and  on  this 
style,  below  the  stigma, 
the  pollen  has  been  de- 
posited and  is  rubbed 
off  against  the  insect. 
At  the  next  flower 
visited  the  stigma  is 
likely  to  strike  the  pol- 
len obtained  from  the  previous  flower,  and  the  style  will 
deposit  a  new  supply  of  pollen. 

In  the  flower  of  the  common  flag  (see  Fig.  132)  the  nectar 
is  deposited  in  a  pit  at  the  bottom  of  the  chamber  formed 
by  each  style  and  petal.  In  this  chamber  the  stamen  is 
found,  and  more  or  less  roofing  it  over  is  the  flap,  or  shelf, 


Fi<j.  138.  Flower  of  Cypripedium,  showing  the 
flap  overhanging  the  opening  of  the  pouch, 
into  which  a  bee  is  crowding  its  way.  The 
small  figure  to  the  right  shows  a  side  view  of 
the  flap  ;  that  to  the  left  a  view  beneath  the 
flap,  showing  the  two  dark  anthers,  and  be- 
tween them,  further  down  (forward),  the 
stigma  surface. — After  Gibson. 


134 


PLANT   RELATIONS. 


upon  the  upper  surface  of  which  the  stigma  is  developed. 
As  the  insect  crowds  its  way  into  this  narrowing  chamber, 
its  body  is  dusted  by  the  pollen,  and  as  it  visits  the  next 
flower  and  thrusts  aside  the  stigmatic  shelf,  it  is  apt  to 
deposit  upon  it  some  of  the  pollen  previously  received. 

The  story  of  pollination  in  connection  with  the  orchids 
is  still  more  complicated  (see  Fig.  133).  Taking  an  ordi- 
nary orchid  for  illustration,  the  details  are  as  follows.  Each 
of  the  two  pollen  masses  terminates  in  a  sticky  disk  or 
button  ;  between  them  extends  the  concave  stigma  sur- 
face, at  the  bottom  of  which  is  the  opening  into  the  long 
tube-like  spur  in  which  the  nectar  is 
found.  Such  a  flower  is  adapted  to 
the  large  moths,  with  long  probosces 
which  can  reach  the  bottom  of  the 
tube.  As  the  moth  thrusts  its  pro- 
boscis into  the  tube,  its  head  touches 
the  sticky  button  on  each  side,  so  that 
when  it  flies  away  these  buttons  stick 
to  its  head,  sometimes  directly  to  its 
eyes,  and  the  pollen  masses  are  torn 
out.  These  masses  are  then  carried 
to  the  next  flower  and  are  thrust 
against  the  stigma  in  the  attempt  to  get  the  nectar. 

In  the  lady-slipper  (Cypripedium),  another  orchid,  the 
flowers  have  a  conspicuous  pouch  (see  Fig.  137),  in  which 
the  nectar  is  secreted.  A  peculiar  structure,  like  a  flap, 
overhangs  the  opening  of  the  pouch,  beneath  which  are  the 
two  anthers,  and  between  them  the  stigmatic  surface  (see 
Fig.  138).  Into  the  pouch  a  bee  crowds  its  way  and  be- 
comes imprisoned  (see  Fig.  139).  The  nectar  which  the 
bee  obtains  is  in  the  bottom  of  the  pouch  (see  Fig.  140). 
When  escaping,  the  bee  moves  towards  the  opening  over- 
hung by  the  flap  and  rubs  first  against  the  stigmatic  sur- 
face (see  Fig.  141),  and  then  against  the  anthers,  receiving 
pollen  on  its  back  (see  Fig.  142).     A  visit  to  another  flower 


Fig.  139.  A  bee  imprisoned 
in  the  pouch  (partly  cut 
away)  of  Cypripedium. 
—After  Gibson. 


FLOWERS   AND   INSECTS. 


135 


Fig.  140. 


A  bee  obtaining  nectar  in  the    pouch  of 
C'ypripedium.—  After  Gibson. 


will  result  in  rubbing  some  of  the  pollen  upon  the  stigma, 
and  in  receiving  more  pollen  for  another  flower. 

In  cases  of  protandry,  as  the  common  figwort,  flowers 
in  the   two  condi- 
tions will  be  visited 
by  the  pollinating 
insect,  and  as  the 
shedding    stamens 
and  receptive  stig- 
mas   occupy    the 
same  relative  posi- 
tion,   the    pollen 
from    one    flower 
will  be  carried  to  the  stigma  of  another.     It  is  evident  that 
exactly  the  same  methods  prevail  in  the  case  of  protogyny, 
as  the  fire  weed  (see  Fig.  134). 

The  Houstonia  (see  Fig.  135),  in  which  there  are  sta- 
mens and  styles  of  different  lengths,  is  visited  by  insects 

whose  bodies  fill 
the  tube  and  pro- 
trude above  it.  In 
visiting  flowers  of 
both  kinds,  one  re- 
gion of  the  body 
receives  pollen 
from  the  short  sta- 
mens, and  another 
region  from  the 
long  stamens.  In 
this  way  the  insect 
will  carry  about  two  bands  of  pollen,  which  come  in  con- 
tact with  the  corresponding  stigmas.  When  there  are  three 
forms  of  flowers,  as  mentioned  in  the  case  of  one  of  the 
loosestrifes,  the  insect  receives  three  pollen  bands,  one  for 
each  of  the  three  sets  of  stigmas. 

93.  Warding  off  unsuitable  insects. — Prominent   among 


Fig.  141.  A  bee  escaping  from  the  pouch  of  Cypri- 
pedium,  and  coming  in  contact  with  the  stigma. 
Advancing  a  little  further  the  bee  will  come  in  con- 
tact with  the  anthers  and  receive  pollen. — After 
Gibson. 


136 


PLANT   RELATIONS. 


the  unsuitable  insects,  which  Kerner  calls  "  unbidden 
guests,"  are  ants,  and  adaptations  for  reducing  their  visits 
to  a  minimum  may  be  taken  as  illustrations. 

(1)  Hairs. — A  common  device  for  turning  back  ants, 
and  other  creeping  insects,  is  a  barrier  of  hair  on  the  stem, 
or  in  the  flower  cluster,  or  in  the  flower. 

(2)  Glandular  secretions. — In  some  cases  a  sticky 
secretion   is   exuded   from   the   surface   of   plants,    which 

effectively  stops 
the  smaller  creep- 
ing insects.  In 
certain  species  of 
catch-fly  a  sticky 
ring  girdles  each 
joint  of  the  stem. 

(3)  Isolation. — 
The  leaves  of  cer- 
tain plants  form 
water  reservoirs 
about  the  stem. 
To  ascend  such  a 
stem,  therefore,  a 
creeping  insect 
must  cross  a  series 
of  such  reservoirs. 
Teasel  furnishes  a 
common  illustration,  the  opposite  leaves  being  united  at 
the  base  and  forming  a  series  of  cups.  More  extensive 
water  reservoirs  are  found  in  Bilbergia  and  Ravenala 
("  traveler's  tree  "),  whose  flower  clusters  are  protected  by 
reservoirs  formed  by  the  rosettes  of  leaves,  which  creeping 
insects  cannot  cross. 

(4)  Latex. — This  is  a  milky  secretion  found  in  some 
plants,  as  in  milkweeds.  Caoutchouc  is  a  latex  secretion 
of  certain  tropical  trees.  When  latex  is  exposed  to  the 
air  it  stiffens  immediately,  becoming  sticky  and  finally 


•^t 


Fig.  142.  A  bee  escaping  from  the  pouch  of  Cypri- 
pedium,  and  rubbing  against  an  anther. — After 
Gibson. 


FLOWERS   AND   INSECTS.  137 

hard.  In  the  flower  clusters  of  many  latex-secreting 
plants  the  epidermis  of  the  stem  is  very  smooth  and  deli- 
cate, and  easily  pierced  by  the  claws  of  ants  and  other 
creeping  insects  who  seek  to  maintain  footing  on  the 
smooth  surface.  Wherever  the  epidermis  is  pierced  the 
latex  gushes  out,  and  by  its  stiffening  and  hardening  glues 
the  insect  fast. 

(5)  Protective  forms. — In  some  cases  the  structure  of 
the  flower  prevents  the  access  of  small  creeping  insects  to 
the  pollen  or  to  the  nectar.  In  the  common  snapdragon 
the  two  lips  are  firmly  closed  (see  Fig.  74),  and  they  can  be 
forced  apart  only  by  some  heavy  insect,  as  the  bumble-bee, 
alighting  upon  the  projecting  lower  lip,  all  lighter  insects 
being  excluded.  In  many  species  of  Pentstemon,  one  of 
the  stamens  does  not  develop  pollen  sacs,  b.ut  lies  like  a  bar 
across  the  mouth  of  the  pit  in  which  the  nectar  is  secreted. 
Through  the  crevices  left  by  this  bar  the  thin  proboscis  of 
a  moth  or  butterfly  can  pass,  but  not  the  whole  body  of  a 
creeping  insect.  Very  numerous  adaptations  of  this  kind 
may  be  observed  in  different  flowers. 

(6)  Protective  closure. —  Certain  flowers  are  closed  at 
certain  hours  of  the  day,  when  there  is  the  chief  danger 
from  creeping  insects.  For  instance,  the  evening  prim- 
roses open  at  dusk,  after  the  deposit  of  dew,  when  ants  are 
not  abroad  ;  and  at  the  same  time  they  secure  the  visits  of 
moths,  which  are  night-fliers. 

Numerous  other  adaptations  to  hinder  the  visits  of 
unsuitable  insects  may  be  observed,  but  those  given  will 
serve  as  illustrations.  In  all  cases  it  must  be  understood 
that  these  so-called  "  adaptations  "  have  not  been  produced 
to  ward  off  insects,  but  that  having  appeared  from  one 
cause  or  another  they  have  proved  to  be  useful  in  this 
particular. 


CHAPTER  VIII. 

AN  INDIVIDUAL  PLANT  IN  ALL  OF  ITS  RELATIONS. 

For  the  purpose  of  summarizing  the  general  life-rela- 
tions detailed  in  the  preceding  chapters,  it  will  be  useful  to 
apply  them  in  the  case  of  a  single  plant.  Taking  a  com- 
mon seed-plant  as  an  illustration,  and  following  its  history 
from  the  germination  of  the  seed,  certain  general  facts 
become  evident  in  its  relations  to  the  external  world. 

94.  Germination  of  the  seed. — The  most  obvious  needs  of 
the  seed  for  germination  are  certain  amounts  of  moisture 
and  heat.  In  order  to  secure  these  to  the  best  advantage, 
the  seed  is  usually  very  definitely  related  to  the  soil,  either 
upon  it  and  covered  by  moisture  and  heat-retaining  debris, 
or  embedded  in  it.  Along  with  the  demand  for  heat  and 
moisture  is  one  for  air  (supplying  oxygen),  which  is  essen- 
tial to  life.  The  relation  which  germinating  seeds  need, 
therefore,  is  one  which  not  only  secures  moisture  and  heat 
advantageously,  but  permits  a  free  circulation  of  air. 

95.  Direction  of  the  root. — The  first  part  of  the  young 
plantlet  to  emerge  from  the  seed  is  the  tip  of  the  axis 
which  is  to  develop  the  root  system.  It  at  once  shows  a 
response  to  the  earth  influence  (geotropism)  and  to  the 
moisture  influence  (hydrotropism),  for  whatever  the  direc- 
tion of  emergence  from  the  seed,  a  curvature  is  developed 
which  directs  the  tip  towards  and  finally  into  the  soil  (see 
Fig.  143).  When  the  soil  is  penetrated  the  primary  root 
may  continue  to  grow  vigorously  downward,  showing  a 
strong  geotropic  tendency,  and  forming  what  is  known 
as  the  tap-root,  from  which  lateral  roots  arise,  which  are 

138 


AN   INDIVIDUAL   PLANT   IN   ALL   OF  ITS   RELATIONS.     139 


much  more  influenced  in  direction  by  other  external 
causes,  especially  the  presence  of  moisture.  As  a  rule, 
the  soil  is  not  perfectly  uniform,  and  contact  with  different 
substances  induces  curvatures,  and  as  a  result  of  these  and 
other  causes,  the  root  system  may  become  very  intricate, 
which   is  extremely  favor-  -p  „ 

able   for   absorbing   and 
gripping. 

96.  Direction  of  the  stem. 
— As  soon  as  the  stem  tip 
is  extricated  from  the  seed, 
it  shows  a  response  to  the 
light  influence  (heliotro- 
pism),  being  guided  in  a 
general  way  towards  the 
light  (see  Fig.  143a). 
Direction  towards  the 
light,  the  source  of  the  in- 
fluence, is  spoken  of  as 
positive  heliotropism,  as 
distinguished  from  direc- 
tion away  from  the  light, 
called  negative  heliotro- 
pism. If  the  main  axis 
continues  to  develop,  it 
continues  to  show  this  posi- 
tive heliotropism  strongly, 
but  the  branches  may  show 
every  variation  from  positive  to  transverse  heliotropism  ; 
that  is,  a  direction  transverse  to  the  direction  of  the  rays 
of  light.  In  some  plants  certain  stems,  as  stolons,  run- 
ners, etc.,  show  strong  transverse  heliotropism,  while  other 
stems,  as  rootstocks,  etc.,  show  a  strong  transverse  geot- 
ropism. 

97.  Direction  of  foliage  leaves. — The  general  direction  of 
foliage  leaves  on  an  erect  stem  is  transversely  heliotropic  ; 


Fig.  143.  Germination  of  the  seed  of 
arbor-vitse  (Tkaja).  B  shows  the 
emergence  of  the  axis  (?•)  which  is  to 
develop  the  root,  and  its  turning  to- 
wards the  soil.  C  shows  a  later  stage, 
in  which  the  root  (r)  has  been  some- 
what developed,  and  the  stem  of  the 
embryo  (Jl)  is  developing  a  curve  pre- 
paratory to  pulling  out  the  seed  leaves 
(cotyledons).  E  shows  the  young  plant- 
let  entirely  free  from  the  seed,  with  its 
root  (/■)  extending  into  the  soil,  its  stem 
(h)  erect,  and  its  first  leaves  (c)  hori- 
zontally spread.— After  Strasburger. 


140 


PLANT  RELATIONS. 


if  necessary,  the  parts  of  the  leaf  or  the  stem  itself  twisting 
to  allow  the  blade  to  assume  this  position.  The  danger  of 
the  leaves  shading  one  another  is  reduced  to  a  minimum  by 
the  elongation  of  internodes,  the  spiral  arrangement,  short- 
ening and  changing  direction  upwards,  or  lobing. 

This  outlines  the  general  nutritive  relations,  the  roots 


Fig.  143a.  Germination  of  the  garden  bean,  showing  the  arch  of  the  seedling  stem 
above  ground,  its  pull  on  the  seed  to  extricate  the  cotyledons  and  plumule,  and 
the  final  straightening  of  the  stem  and  expansion  of  the  young  leaves. — After 
Atkinson. 


and  leaves  being  favorably  placed  for  absorption,  and  the 
latter  also  favorably  placed  for  photosynthesis.  It  is  im- 
portant to  study  the  behavior  of  various  plants  in  the 
germination  of  the  seed,  for  in  a  comparatively  short  period 
all  of  the  important  external  relations  of  the  vegetative 
organs  are  established.  Seeds  should  be  selected  that  ger- 
minate rapidly,  and  that  represent  different  great  groups, 
such  as  squash,  bean,  corn,  etc.,  and  these  observations 
should  be  extended  as  far  as  possible  by  including  the  obser- 
vation of  seedlings  in  nature. 


AN  INDIVIDUAL   PLANT   IN   ALL   OF  ITS  RELATIONS.    141 

98.  Placing  of  flowers. — The  purposes  of  the  flower  seem 
to  be  served  best  by  exposed  positions,  and  consequently 
flowers  appear  mostly  at  the  extremities  of  stems  and 
branches,  a  position  evidently  favorable  to  pollination  and 
seed  dispersal.  The  flowers  thus  exposed  are  very  com- 
monly massed,  or,  if  not,  the  single  flower  is  apt  to  be  large 
and  conspicuous.  The  various  devices  for  protecting  nec- 
tar and  pollen  against  too  great  moisture,  and  the  more 
delicate  structures  against  chill ;  for  securing  the  visits  of 
suitable  insects,  and  warding  off  unsuitable  insects ;  and 
for  dispersing  the  seeds,  need  not  be  repeated. 

99.  Branch  buds. — If  the  plant  under  examination  be  a 
tree  or  shrub,  branch  buds  will  be  observed  to  be  developed 
during  the  growing  season  (see  Fig.  65).  This  device  for 
protecting  growing  tips  through  a  season  of  dangerous  cold 
is  very  familiar  to  those  living  in  the  temperate  regions. 
The  internodes  do  not  elongate,  hence  the  leaves  overlap; 
they  develop  little  or  no  chlorophyll,  and  become  scales. 
The  protection  afforded  by  these  overlapping  scales  is  often 
increased  by  the  development  of  hairs,  or  by  the  secretion 
of  mucilage  or  gum. 


CHAPTER  IX. 

THE   STRUGGLE  FOR  EXISTENCE. 

100.  Definition. — The  phrase  "struggle  for  existence" 
has  come  to  mean,  so  far  as  plants  are  concerned,  that  it  is 
usually  impossible  for  them  to  secure  ideal  relations,  and 
that  they  must  encounter  unfavorable  conditions.  The 
proper  light  and  heat  relations  may  be  difficult  to  obtain, 
and  also  the  proper  relations  to  food  material.  It  often 
happens,  also,  that  conditions  once  fairly  favorable  may  be- 
come unfavorable.  Also,  multitudes  of  plants  are  trying 
to  take  ]30ssession  of  the  same  conditions.  All  this  leads 
to  the  so-called  "struggle,"  and  vastly  more  plants  fail 
than  succeed.  Before  considering  the  organization  of  plant 
associations,  it  will  be  helpful  to  consider  some  of  the 
possible  changes  in  conditions,  and  the  effect  on  plants. 

101.  Decrease  of  water. — This  is  probably  the  most  com- 
mon factor  to  fluctuate  in  the  environment  of  a  plant. 
Along  the  borders  of  streams  and  ponds,  and  in  swampy 
places,  the  variation  in  the  water  is  very  noticeable,  but  the 
same  thing  is  true  of  soils  in  general.  However,  the  change 
chiefly  referred  to  is  that  which  is  permanent,  and  which 
compels  plants  not  merely  to  tide  over  a  drouth,  but  to 
face  a  permanent  decrease  in  the  water  supply. 

Around  the  margins  of  ponds  are  very  commonly  seen 
fringes  of  such  plants  as  bulrushes,  cat-tail  flags,  reed- 
grasses,  etc.,  standing  in  shoal  water.  As  these  plants 
partially  decay,  their  bodies  and  the  entangled  silt  from 
the  land  presently  accumulate  to  such  an  extent  that  there 
is  no  more  standing  water,  and  the  water  supply  for  the 


THE    STRUGGLE    FOR   EXISTENCE.  143 

bulrushes  and  their  associates  has  permanently  decreased 
below  the  favorable  amount.  In  this  way  certain  lake 
margins  gradually  encroach  upon  the  water,  and  in  so 
doing  the  water  supply  is  permanently  diminished  for  many 
plants.  By  the  same  process,  smaller  lakelets  are  gradually 
being  converted  into  bogs,  and  the  bogs  in  turn  into  drier 
ground,  and  these  unfavorable  changes  in  water  supply  are 
a  menace  to  many  plants. 

The  operations  of  man,  also,  have  been  very  effective  in 
diminishing  the  water  supply  for  plants.  Drainage,  which 
is  so  extensively  practiced,  while  it  may  make  the  water- 
supply  more  favorable  for  the  plants  which  man  desires,  cer- 
tainly makes  it  very  unfavorable  for  many  other  plants. 
The  clearing  of  forests  has  a  similar  result.  The  forest 
soil  is  receptive  and  retentive  in  reference  to  water,  and  is 
somewhat  like  a  great  sponge,  steadily  supplying  the  streams 
which  drain  it.  The  removal  of  the  forest  destroys  much 
of  this  power.  The  water  is  not  held  and  gradually  doled 
out,  but  rushes  off  in  a  flood  ;  hence,  the  streams  which 
drain  the  cleared  area  are  alternately  flooded  and  dried  up. 
This  results  in  a  much  less  total  supply  of  water  available 
for  the  use  of  plants. 

102.  Decrease  of  light. — It  is  very  common  to  observe 
tall,  rank  vegetation  shading  lower  forms,  and  seriously 
interfering  with  the  light  supply.  If  the  rank  vegetation 
is  rather  temporary,  the  low  plants  may  learn  to  precede  or 
follow  it,  and  so  avoid  the  shading  ;  but  if  the  over-shading 
vegetation  is  a  forest  growth,  shading  becomes  permanent. 
In  the  case  of  deciduous  trees,  which  drop  their  leaves  at  the 
close  of  the  growing  season  and  put  out  a  fresh  crop  in  the 
spring,  there  is  an  interval  in  the  early  spring,  before  the 
leaves  are  fully  developed,  during  which  low  plants  may 
secure  a  good  exposure  to  light  (see  Fig.  144).  In  such 
places  one  finds  an  abundance  of  "spring  flowers,"  but  later 
in  the  season  the  low  plants  become  very  scarce.  This 
effective  over-shading  is  not   common   to   all   forests,  for 


Pig.  144.  A  common  spring  plant  (dog-tooth  violet)  which  grows  in  deciduous 
forests.  The  large  mottled  leaves  and  the  conspicuous  flowers  are  sent  rapidly 
above  the  surface  from  the  subterranean  bulb  (see  cut  in  the  left  lower  corner) 
where  are  also  seen  dissected  out  some  petals  and  stamens  and  the  pistil. 


THE   STRUGGLE   FOR   EXISTENCE.  145 

there  are  "light  forests,"  such  as  the  oak  forest,  which 
permit  much  low  vegetation,  as  well  as  the  shade  forests, 
such  as  beech  forests,  which  permit  very  little. 

In  the  forest  regions  of  the  tropics,  however,  the  shad- 
ing is  jaermanent,  since  there  is  no  annual  fall  of  leaves. 
In  such  conditions  the  climbing  habit  has  been  extensively 
cultivated. 

103.  Change  in  temperature. — In  regions  outside  of  the 
tropics  the  annual  change  of  temperature  is .  a  very  im- 
portant factor  in  the  life  of  plants,  and  they  have  provided 
for  it  in  one  way  or  another.  In  tracing  the  history  of 
plants,  however,  back  into  what  are  called  "geological 
times,"  we  discover  that  there  have  been  relatively  per- 
manent changes  in  temperature.  Now  and  then  glacial 
conditions  prevailed,  during  which  regions  before  temperate 
or  even  tropical  were  subjected  to  arctic  conditions.  It  is 
very  evident  that  such  permanent  changes  of  temperature 
must  have  had  an  immense  influence  upon  plant  life. 

104.  Change  in  soil  composition. — One  of  the  most  ex- 
tensive agencies  in  changing  the  compositions  of  soils  in 
certain  regions  has  been  the  movement  of  glaciers  of  conti- 
nental extent,  which  have  deposited  soil  material  over  very 
extensive  areas.  Areas  within  reach  of  occasional  floods, 
also,  may  have  the  soil  much  changed  in  character  by  the 
new  deposits.  Shifting  dunes  are  billow-like  masses  of 
sand,  developed  and  kept  in  motion  by  strong  prevailing 
winds,  and  often  encroach  upon  other  areas.  Besides  these 
changes  in  the  character  of  soil  by  natural  agencies,  the 
various  operations  of  man  have  been  influential.  Clearing, 
draining,  fertilizing,  all  change  the  character  of  the  soil, 
both  in  its  chemical  composition  and  its  physical  properties. 

105.  Devastating  animals. — The  ravages  of  animals  form 
an  important  factor  in  the  life  of  many  plants.  For  example, 
grazing  animals  are  wholesale  destroyers  of  vegetation,  and 
may  seriously  affect  the  plant  life  of  an  area.  The  various 
leaf  feeders  among  insects   have   frequently  done  a  vast 


146  PLANT   RELATIONS. 

amount  of  damage  to  plants.  Many  burrowing  animals 
attack  subterranean  parts  of  plants,  and  interfere  seriously 
with  their  occupation  of  an  area. 

Various  protective  adaptations  against  such  attacks  have 
been  pointed  out,  but  this  subject  probably  has  been  much 
exaggerated.  The  occurrence  of  hairs,  prickles,  thorns, 
and  spiny  growths  upon  many  plants  may  discourage  the 
attacks  of  animals,  but  it  would  be  rash  to  assume  that 
these  protections  have  been  developed  because  of  the  danger 
of  such  attacks.  One  of  the  families  of  plants  most  com- 
pletely protected  in  this  way  is  the  great  cactus  family, 
chiefly  inhabiting  the  arid  regions  of  southwestern  United 
States  and  Mexico.  In  such  a  region  succulent  vegetation 
is  at  a  premium,  and  it  is  doubtless  true  that  the  armor  of 
thorns  and  bristles  reduces  the  amount  of  destruction. 

In  addition  to  armor,  the  acrid  or  bitter  secretions  of 
certain  plants  or  certain  parts  of  plants  would  have  a 
tendency  to  ward  off  the  attacks  of  animals. 

106.  Plant  rivalry. — It  is  evident  that  there  must  be 
rivalry  among  plants  in  occupying  an  area,  and  that  those 
plants  which  can  most  nearly  utilize  identical  conditions 
will  be  the  most  intense  rivals.  For  example,  a  great  many 
young  oaks  may  start  up  over  an  area,  and  it  is  evident 
that  the  individuals  must  come  into  sharp  competition  with 
one  another,  and  that  but  few  of  them  succeed  in  establish- 
ing themselves  permanently.  This  is  rivalry  between  in- 
dividuals of  the  same  kind  ;  but  some  other  kind  of  trees, 
as  the  beech,  may  come  into  competition  with  the  oak,  and 
another  form  of  rivalry  will  appear. 

As  a  consequence  of  plant  rivalry,  the  different  plants 
which  finally  succeed  in  taking  possession  of  an  area  are 
apt  to  be  dissimilar,  and  a  plant  association  is  usually  made 
up  of  plants  which  represent  widely  different  regions  of  the 
plant  kingdom.  It  is  sometimes  said  that  any  well-devel- 
oped plant  association  is  an  epitome  of  the  plant  kingdom. 

A  familiar  illustration  of  plant  rivalry  may  be  observed 


THE    STRUGGLE    FOR   EXISTENCE.  147 

in  the  case  of  what  are  called  "weeds."  Every  one  is  fa- 
miliar with  the  fact  that  if  cultivated  ground  is  neglected 
these  undesirable  plants  will  invade  it  vigorously  and  seri- 
ously affect  the  development  of  plants  under  cultivation. 

107.  Adaptation. — When  the  changes  mentioned  above 
occur  in  the  environment  of  plants  to  such  an  extent  as 
to  make  the  conditions  for  living  very  unfavorable,  one 
of  three  things  is  likely  to  occur,  adaptation,  migration, 
or  destruction. 

The  change  in  conditions  may  come  slowly  enough,  and 
certain  plants  may  be  able  to  endure  it  long  enough  to 
adjust  themselves  to  it.  Such  an  adjustment  may  involve 
changes  in  structure,  and  probably  no  plants  are  plastic 
enough  to  adjust  themselves  to  extreme  and  sudden  changes 
which  are  to  be  comparatively  permanent.  There  are 
plants,  such  as  the  common  cress,  which  may  be  called 
amphibious,  which  can  live  in  the  water  or  out  of  it  without 
change  of  structure,  but  this  is  endurance  rather  than 
adaptation.  Many  plants,  however,  can  pass  slowly  into 
different  conditions,  such  as  drier  soil,  denser  shade,  etc., 
and  corresponding  changes  in  their  structure  may  be  noted. 
Very  often,  however,  such  plants  are  given  no  opportunity 
to  adjust  themselves  to  the  new  conditions,  as  the  area  is 
apt  to  be  invaded  by  plants  already  better  adapted.  While 
adaptation  may  be  regarded  as  a  real  result  of  changed  con- 
ditions, it  would  seem  to  be  by  no  means  the  common  one. 

108.  Migration. — This    is    a    very    common    result    of 

changed  conditions.     Plants  migrate  as  truly  as  animals, 

though,  of  course,  their  migration  is  from  generation  to 

generation.     It  is  evident,  however,  that  migration  cannot 

be  universal,  for  barriers  of  various  kinds  may  forbid  it. 

In  general,  these  barriers  represent  unfavorable  conditions 

for  living.     If  a  plant  area  with  good  soil  is  surrounded  by 

a  sterile  area,  the  latter  would  form  an  efficient  barrier  to 

migration  from  the  former.     Plants  of  the  lowlands  could 

not  cross  mountains  to  escape  from  unfavorable  conditions. 
11 


148  PLANT  KELATIONS. 

To  make  migration  possible,  therefore,  it  is  necessary  for 
the  conditions  to  be  favorable  for  the  migrating  plants  in 
some  direction.  In  the  case  of  bulrushes,  cat-tail  flags, 
etc.,  growing  in  the  shoal  water  of  a  lake  margin,  the 
building  up  of  soil  about  them  results  in  unfavorable  con- 
ditions. As  a  consequence,  they  migrate  further  into  the 
lake.  If  the  lake  happens  to  be  a  small  one,  the  filling  up 
process  may  finally  obliterate  it,  and  a  time  will  come  when 
such  forms  as  bulrushes  and  flags  will  find  it  impossible  to 
migrate. 

In  glacial  times  very  many  arctic  plants  migrated  south- 
ward, especially  along  the  mountain  systems,  and  many 
alpine  plants  moved  to  lower  ground.  When  warmer  con- 
ditions returned,  many  plants  that  had  been  driven  south 
returned  towards  the  north,  and  the  arctic  and  alpine  plants 
retreated  to  the  north  and  up  the  mountains.  The  history 
of  plants  is  full  of  migrations,  compelled  by  changed  con- 
ditions and  permitted  in  various  directions.  It  must  be 
remembered,  also,  that  migrations  often  result  in  changes 
of  structure. 

109.  Destruction. — Probably  this  is  by  far  the  most  com- 
mon result  of  greatly  changed  conditions.  Even  if  plants 
adapt  themselves  to  changed  conditions,  or  migrate,  their 
structure  may  be  so  changed  that  they  will  seem  like  quite 
different  plants.  In  this  way  old  forms  gradually  disappear 
and  new  ones  take  their  places. 


CHAPTER  X. 

THE   NUTRITION   OP   PLANTS. 

110.  Physiology. — In  the  previous  chapters  plants  have 
been  considered  in  reference  to  their  surroundings.  It 
was  observed  that  various  organs  of  nutrition  hold  certain 
life-relations,  but  it  is  essential  to  discover  what  these  rela- 
tions mean  to  the  life  of  the  plant.  The  study  of  plants 
from  the  standpoint  of  their  life-relations  has  been  called 
Ecology  ;  the  study  of  the  life-processes  of  plants  is  called 
Physiology.  These  two  points  of  view  may  be  illustrated 
by  comparing  them  to  two  points  of  view  for  the  study  of 
man.  Man  may  be  studied  in  reference  to  his  relation  to 
his  fellow-men  and  to  the  character  of  the  country  in  which 
he  lives  ;  or  his  bodily  processes  may  be  studied,  such  as 
digestion,  circulation,  respiration,  etc.  The  former  cor- 
responds to  Ecology,  the  latter  is  Physiology. 

All  of  the  ecological  relations  that  have  been  mentioned 
find  their  meaning  in  the  physiology  of  the  plant,  for  life- 
relations  have  in  view  life-processes.  The  subject  of  plant 
physiology  is  a  very  complex  one,  and  it  would  be  impossi- 
ble in  an  elementary  work  to  present  more  than  a  few  very 
general  facts.  Certain  facts  in  reference  to  plant  move- 
ments, an  important  physiological  subject,  have  been  men- 
tioned in  connection  with  life-relations,  but  it  seems  neces- 
sary to  make  some  special  mention  of  nutrition. 

111.  Significance  of  chlorophyll. — Probably  the  most  im- 
portant fact  to  observe  in  reference  to  the  nutrition  of 
plants  is  that  some  plants  are  green  or  have  green  parts, 
while  others,  such  as  toadstools,  do  not  show  this  green 


150  PLANT   EELATIONS. 

color.  It  has  been  stated  that  this  green  color  is  due  to 
the  presence  of  a  coloring  matter  known  as  chlorophyll 
(see  §12).  The  two  groups  may  be  spoken  of,  therefore, 
as  (1)  green  plants  and  (2)  plants  without  chlorophyll. 
The  presence  of  chlorophyll  makes  it  possible  for  the  plants 
containing  it  to  manufacture  their  own  food  out  of  such 
materials  as  water,  soil  material,  and  gases.  For  this 
reason,  green  plants  may  be  entirely  independent  of  all 
other  living  things,  so  far  as  their  food  supply  is  concerned. 

Plants  without  chlorophyll,  however,  are  unable  to 
manufacture  food  out  of  such  materials,  and  must  obtain 
it  already  manufactured  in  the  bodies  of  other  plants  or 
animals.  For  this  reason,  they  are  dependent  upon  other 
living  things  for  their  food  supply,  just  as  are  animals.  It 
is  evident  that  plants  without  chlorophyll  may  obtain  this 
food  supply  either  from  the  living  bodies  of  plants  and  ani- 
mals, in  which  case  they  are  called  parasites,  or  they  may 
obtain  it  from  the  substances  derived  from  the  bodies  of 
plants  and  animals,  in  which  case  they  are  called  sapro- 
phytes. For  example,  the  rust  which  attacks  the  wheat, 
and  is  found  upon  the  leaves  and  stems  of  the  living  plant, 
is  a  parasite ;  while  the  mould  which  often  develops  on  stale 
bread  is  a  saprophyte.  Some  plants  without  chlorophyll 
can  live  either  as  parasites  or  saprophytes,  while  others  are 
always  one  or  the  other.  By  far  the  largest  number  of 
parasites  and  saprophytes  belong  to  the  group  of  low  plants 
called  fungi,  and  when  fungi  are  referred  to,  it  must  be 
understood  that  it  means  the  greatest  group  of  plants  with- 
out chlorophyll. 

112.  Photosynthesis. — The  nutritive  processes  in  green 
plants  are  the  same  as  mother  plants,  and  in  addition  there 
is  in  green  plants  the  peculiar  process  known  as  photosyn- 
thesis (see  §25).  In  plants  with  foliage  leaves,  these  are 
the  chief  organs  for  this  work.  It  must  be  remembered, 
however,  that  leaves  are  not  necessary  for  photosynthesis, 
for  plants  without  leaves,  such  as  algaj,  perform  it.      The 


THE   NUTRITION   OF   PLANTS.  151 

essential  thing  is  green  tissue  exposed  to  light,  but  in  this 
brief  account  an  ordinary  leafy  plant  growing  in  the  soil 
will  be  considered. 

As  the  leaves  are  the  active  structures  in  the  work  of 
photosynthesis,  the  raw  materials  necessary  must  be  brought 
to  them.  In  a  general  way,  these  materials  are  carbon  di- 
oxide and  water.  The  gas  exists  diffused  through  the 
atmosphere,  and  so  is  in  contact  with  the  leaves.  It  also 
occurs  dissolved  in  the  water  of  the  soil,  but  the  gas  used 
is  absorbed  from  the  air  by  the  leaves.  The  supply  of 
water,  on  the  other  hand,  in  soil-related  plants,  is  obtained 
from  the  soil.  The  root  system  absorbs  this  water,  which 
then  ascends  the  stem  and  is  distributed  to  the  leaves. 

(1)  Ascent  of  water. — The  water  does  not  move  up- 
wards through  all  parts  of  the  stem,  but  is  restricted  to  a 
certain  definite  region.  This  region  is  easily  recognized  as 
the  woody  part  of  stems.  Sometimes  separate  strands  of 
wood,  looking  like  fibers,  may  be  seen  running  lengthwise 
through  the  stem  ;  sometimes  the  fibrous  strands  are  packed 
so  close  together  that  they  form  a  compact  woody  mass,  as 
in  shrubs  and  trees.  In  the  case  of  most  trees  new  wood  is 
made  each  year,  through  which  the  water  moves.  Hence 
the  very  common  distinction  is  made  between  sap-wood, 
through  which  the  water  is  moving,  and  heart-wood,  which 
the  water  current  has  abandoned.  Just  how  the  water 
ascends  through  these  woody  fibers,  especially  in  tall  trees, 
is  a  matter  of  much  discussion,  and  cannot  be  regarded  as 
definitely  known.  In  any  event,  it  should  be  remembered 
that  these  woody  fibers  are  not  like  the  open  veins  and 
arteries  of  animal  bodies,  and  no  "circulation"  is  possible. 
These  same  woody  strands  are  seen  branching  throughout 
the  leaves,  forming  the  so-called  vein  system,  and  it  is  evi- 
dent, therefore,  that  they  form  a  continuous  route  from 
roots  to  leaves. 

It  is  easy  to  demonstrate  the  ascent  of  water  in  the 
stem,  and  the  path  it  takes,  by  a  simple  experiment.     If 


152  PLANT   RELATIONS. 

an  active  stem  be  cut  and  plunged  into  water  stained  with 
an  aniline  color  called  eosin,*  the  ascending  water  will  stain 
its  pathway.  After  some  time  sections  through  the  stem 
will  show  that  the  water  has  traveled  upwards  through  it, 
and  the  stain  will  point  out  the  region  of  the  stem  used  in 
the  movement. 

In  general,  therefore,  the  carbon  dioxide  is  absorbed 
directly  from  the  air  by  the  leaves,  and  the  water  is  ab- 
sorbed by  the  root  from  the  soil,  and  moves  upwards  through 
the  stem  into  the  leaves.  An  interesting  fact  about  these 
raw  materials  is  that  they  are  very  common  waste  products. 
They  are  waste  products  because  in  most  life-processes  they 
cannot  be  taken  to  pieces  and  used.     The  fact  that  they 

can  be  used  in  photosynthesis 
shows  that  it  is  a  very  re- 
markable life  process. 

(2)  Chloroplasts. — Having 
obtained  some  knowledge  of 
the  raw  materials  used  in 
photosynthesis,    and    their 

Fig.  145.    Some  mesopnyll  cells  from      x  .         . 

the  leaf  of  i^Monia,  showing  chloro-      SOlirceS,      it      is     necessary     to 

plasts-  consider  the  plant  machinery 

arranged  for  the  work.  In  the  working  leaf  cells  it  is 
discovered  that  the  color  is  due  to  the  presence  of  very 
small  green  bodies,  known  as  chlorophyll  bodies  or  chloro- 
plasts (see  Fig.  145).  These  consist  of  the  living  substance, 
known  as  protoplasm,  and  the  green  stain  called  chloro- 
phyll ;  therefore,  each  chloroplast  is  a  living  body  (plastid) 
stained  green.  It  is  in  these  chloroplasts  that  the  work  ol 
photosynthesis  is  done.  In  order  that  they  may  work  it 
is  necessary  for  them  to  obtain  a  supply  of  energy  from 
some  outside  source,  and  the  source  used  in  nature  is  sun- 
light. The  green  stain  (chlorophyll)  seems  to  be  used  in 
absorbing   the   necessary  energy  from  sunlight,   and   the 

*  The  commoner  grades  of  red  ink  are  usually  solutions  of  eosin. 


THE   NUTRITION   OF   PLANTS.  153 

plastid  uses  this  energy  in  the  work  of  photosynthesis.  It 
is  evident,  therefore,  that  photosynthesis  goes  on  only  in 
the  sunlight,  and  is  suspended  entirely  at  night.  It  is 
found  that  any  intense  light  can  be  used  as  a  substitute 
for  sunlight,  and  plants  have  been  observed  to  carry  on 
the  work  of  photosynthesis  in  the  presence  of  electric 
light. 

(3)  Result  of  photosynthesis. — The  result  of  this  work 
can  be  stated  only  in  a  very  general  way.  Carbon  dioxide 
is  composed  of  two  elements,  carbon  and  oxygen,  in  the 
proportion  one  part  of  carbon  to  two  parts  of  oxygen. 
Water  is  also  composed  of  two  elements,  hydrogen  and  oxy- 
gen. In  photosynthesis  the  elements  composing  these  sub- 
stances are  separated  from  one  another,  and  recombined  in 
a  new  way.  In  the  process  a  certain  amount  of  oxygen  is 
liberated,  just  as  much  as  was  in  the  carbon  dioxide,  and  a 
new  substance  is  formed,  known  as  a  carbohydrate.  The 
oxygen  set  free  escapes  from  the  plant,  and  may  be  re- 
garded as  waste  product  in  the  process  of  photosynthesis. 
It  will  be  remembered  that  the  external  changes  in  this 
process  are  the  absorption  of  carbon  dioxide  and  the  giving 
off  of  oxygen  (see  §25). 

(4)  Carbohydrates  and  proteids.  —  The  carbohydrate 
formed  is  an  organic  substance ;  that  is,  a  substance  made 
in  nature  only  by  life  processes.  It  is  the  same  kind  of 
substance  as  sugar  or  starch,  and  all  are  known  as  carbohy- 
drates ;  that  is,  substances  composed  of  carbon,  and  of  hy- 
drogen and  oxygen  in  the  same  proportion  as  in  water. 
The  work  of  photosynthesis,  therefore,  is  to  form  carbohy- 
drates. The  carbohydrates,  such  as  sugar  and  starch,  rep- 
resent but  one  type  of  food  material.  Proteids  represent 
another  prominent  type,  substances  which  contain  carbon, 
hydrogen,  and  oxygen,  as  do  carbohydrates,  but  which  also 
contain  other  elements,  notably  nitrogen,  sulphur,  and 
phosphorus.  The  white  of  an  egg  may  be  taken  as  an  ex- 
ample of  proteids.     They  seem  to  be  made  from  the  carbo- 


154  PLANT   RELATIONS. 

hydrates,  the  nitrogen,  sulphur,  and  other  necessary 
additional  elements  being  obtained  from  soil  substances 
dissolved  in  the  water  which  is  absorbed  and  conveyed 
to  the  leaves. 

113.  Transpiration. — The  water  which  is  absorbed  by  the 
roots  and  passes  to  the  leaves  is  much  more  abundant  than 
is  needed  in  the  process  of  photosynthesis.  It  should  be  re- 
membered that  the  water  is  not  only  used  as  a  raw  material 
for  food  manufacture,  but  also  acts  as  a  solvent  of  the  soil 
materials  that  are  passing  into  the  plant.  The  water  in 
excess  of  the  small  amount  used  in  food  manufacture  is 
given  off  from  the  plant  in  the  form  of  water  vapor,  the 
process  being  already  referred  to  as  transpiration  (see  §26). 

114.  Digestion. — Carbohydrates  and  proteids  may  be  re- 
garded as  prominent  types  of  plant  food  which  green 
plants  are  able  to  manufacture.  These  foods  are  trans- 
ported through  the  plant  to  regions  where  work  is  going  on, 
and  if  there  is  a  greater  supply  of  food  than  is  needed  for 
the  working  regions,  the  excess  is  stored  up  in  some  part 
of  the  plant.  As  a  rule,  green  plants  are  able  to  manufac- 
ture much  more  food  than  they  use,  and  it  is  upon  this  ex- 
cess that  other  plants  and  animals  live.  In  the  transfer  of 
foods  through  the  plant  certain  changes  are  often  neces- 
sary. For  example,  starch  is  insoluble,  and  hence  cannot 
be  carried  about  in  solution.  It  is  necessary  to  transform 
it  into  sugar,  which  is  soluble.  These  changes,  made  to 
facilitate  the  transfer  of  foods,  represent  digestion. 

115.  Assimilation. — When  food  in  some  form  has  reached 
a  working  region,  it  is  organized  into  the  living  substance 
of  the  plant,  known  as  protoplasm,  and  the  protoplasm 
builds  the  plant  structure.  This  process  of  organizing  the 
food  into  the  living  substance  is  known  as  assimilation. 

116.  Respiration. — The  formation  of  foods,  their  diges- 
tion and  assimilation  are  all  preparatory  to  the  process  of 
respiration,  which  may  be  called  the  use  of  assimilated 
food.     The  whole  working  power   of  the   plant   depends 


THE   NUTRITION   OF   PLANTS.  155 

upon  respiration,  which  means  the  absorption  of  oxygen  by 
the  protoplasm,  the  breaking  down  of  protoplasm,  and  the 
giving  off  of  carbon  dioxide  and  water  as  wastes.     The  im- 


Fig,  146.  The  common  Northern  pitcher  plant.  The  hollow  leaves,  each  with  a  hood 
and  a  wing,  form  a  rosette,  from  the  center  of  which  arise  the  flower  stalks.— 
After  Keener. 

portance  of  this  process  may  be  realized  when  it  is  remem- 
bered that  there  is  the  same  need  in  our  own  living,  as  it 
is  essential  for  us  also  to  "  breathe  in  "  oxygen,  and  as  a 
result  we  "  breathe  out  "  carbon  dioxide  and  water.  This 
breaking  down  or  "  oxidizing  "  of  protoplasm  releases  the 


156 


PLANT  RELATIONS. 


power  by  which  the  work  of  the  plant  is  carried  on  (see 
§27). 

117.  Summary  of  life-processes. — To  summarize  the  nu- 
tritive life-processes  in  green  plants,  therefore,  photosyn- 
thesis manufactures  carbohydrates, 
the  materials  used  being  carbon 
dioxide  and  water,  the  work  being 
done  by  the  chloroplast  with  the 
aid  of  light ;  the  manufacture  of 
proteids  uses  these  carbohydrates, 
and  also  substances  containing 
nitrogen,  sulphur,  etc.;  digestion 
puts  the  insoluble  carbohydrates 
and  the  proteids  into  a  soluble 
form  for  transfer  through  the 
plant ;  assimilation  converts  this 
food  material  into  the  living  sub- 
stance of  the  plant,  protoplasm  ; 
respiration  is  the  oxidizing  of  the 
protoplasm  which  enables  the 
plant  to  work,  oxygen  being  ab- 
sorbed, and  carbon  dioxide  and 
water  vapor  being  given  off  in 
the  process. 

118.  Plants  without  chlorophyll. 
— Eemembering  the  life-processes 
described  under  green  plants,  it  is 
evident  that  plants  without  chlo- 

Fig    147      The  Southern  pitcher  ]     H     calmot     do     the    WQrk     of 

plant,  showing  the  funnelform  r    J 

and    winged  pitcher,   and    the    photosynthesis.       This   HieUllS  that 

overarching  hood  with  transia-   they   c.limot    manufacture    carbo- 

cent  spots. — After  Keknek.  j 

hydrates,  and  that  they  must  de- 
pend upon  other  plants  or  animals  for  this  important  food. 
Mushrooms,  puff-balls,  moulds,  mildews,  rusts,  dodder, 
corpse  plants,  beech  drops,  etc.,  may  be  taken  as  illustra- 
tions of  such  plants. 


THE   NUTRITION   OF   PLANTS. 


157 


Although  plants  without  chlorophyll  cannot  manufac- 
ture carbohydrates,  the  other  processes,  proteid  manufac- 
ture, digestion,  assimilation,  and  respiration,  are  carried  on. 
It  is  true,  however,  that  in  obtaining  carbohydrates  from 
other  plants  and  ani- 
mals, proteids  are  ob- 
tained also,  so  that 
proteid  manufacture 
is  not  so  prominent  as 
in  green  plants. 

119.  "Carnivorous" 
plants. — This  name  has 
been  given  to  plants 
which  have  developed 
the  curious  habit  of 
capturing  insects  and 
using  them  for  food. 
They  are  green  plants 
and,  therefore,  can  man- 
ufacture carbohydrates. 
But  they  live  in  soil 
poor  in  nitrogen  com- 
pounds, and  hence  pro- 
teid formation  is  inter- 
fered with.  The  bodies 
of  captured  insects  sup- 
plement the  proteid 
supply,  and  the  plants 
have  come  to  depend 
upon  them.  Many,  if 
not  all  of  these  carniv- 
orous plants,  secrete  a 
digestive  substance 
which    acts    upon    the 

bodies  of  the   captured  insects  very  much  as  the   diges- 
tive substances  of  the  alimentary  canal  act  upon  proteids 


Fig.  148.  The  Californian  pitcher  plant  (Darling- 
torda),  showing  twisted  and  winged  pitcher, 
the  overarching  hood  with  translucent  spots, 
and  the  fish-tail  appendage  to  the  hood 
which  is  attractive  to  flying  insects.— After 
Kerner. 


158 


PLANT   RELATIONS. 


swallowed  by  animals.      Some  common  illustrations  are  as 
follows  : 

(1)  Pitcher  plants. — In  these  plants  the  leaves  form 
tubes,  or  urns,  of  various  forms,  which  contain  water,  and 
to  which  insects  are  attracted  and  drowned  (see  Fig.  146). 
A  pitcher  plant  common  throughout  the  Southern  States 
may  be  taken  as  a  type  (see  Fig.  147).  The  leaves  are 
shaped  like  slender,  hollow  cones,  and  rise  in  a  tuft  from 

the  swampy  ground. 
The  mouth  of  this 
conical  urn  is  over- 
arched and  shaded 
by  a  hood,  in  which 
are  translucent  spots, 
like  small  windows. 
Around  the  mouth 
of  the  urn  are 
glands,  which  se- 
crete a  sweet  liquid 
{nectar),  and  nectar 
drops  form  a  trail 
down  the  outside  of 
the  urn.  Inside,  just 
below  the  rim  of  the 
urn,  is  a  glazed  zone, 
so  smooth  that  insects 
cannot  walk  upon  it. 
Below  the  glazed  zone 
is  another  zone, 
thickly  set  with  stiff, 
downward-pointing  hairs,  and  below  this  is  the  liquid  in 
the  bottom  of  the  urn. 

If  a  fly  is  attracted  by  the  nectar  drops  upon  this  curious 
leaf,  it  naturally  follows  the  trail  up  to  the  rim  of  the  urn, 
where  the  nectar  is  abundant.  If  it  attempts  to  descend 
within  the  urn,  it  slips  on  the  glazed  zone,  and  falls  into 


Fig.   149. 


A  sun-dew,  showing  rosette  habit  of 
the  insect-catching  leaves. 


THE   NUTE1TI0N   OF   PLANTS. 


159 


the  water,  and  if  it  attempts  to  escape  by  crawling  up  the 
sides  of  the  urn,  the  thicket  of  downward-pointing  hairs 
prevents.  If  it  seeks  to  fly  away  from  the  rim,  it  flies 
towards  the  translucent  spots  in  the  hood,  which  look  like 
the  way  of  escape,  as  the  direction  of  entrance  is  in  the 
shadow  of  the  hood.  Pounding  against  the  hood,  the  fly 
falls  into  the  tube.     This  Southern  pitcher  plant  is  known 


Fig.  150.  Two  leaves  of  a  sun-dew.  The  one  to  the  right  has  its  glandular  hairs 
fully  expanded  ;  the  one  to  the  left  shows  half  of  the  hairs  bending  inward,  in  the 
position  assumed  when  an  insect  has  been  captured. — After  Kerner. 


as  a  great  fly-catcher,  and  the  urns  are  often  well  supplied 
with  the  decaying  bodies  of  these  insects. 

A  much  larger  Californian  pitcher  plant  has  still  more 
elaborate  contrivances  for  attracting  insects  (see  Fig.  148). 

(2)  Drosera. — The  droseras  are  commonly  known  as 
" sun-dews,"  and  grow  in  swampy  regions,  the  leaves  form- 
ing small  rosettes  on  the  ground  (see  Fig.  149).  In  one 
form  the  leaf  blade  is  round,  and  the  margin  is  beset  by 
prominent  bristle-like  hairs,  each  with  a  globular  gland  at 
its  tip  (see   Fig.   150).     Shorter   gland-bearing  hairs   are 


160 


PLANT   RELATIONS. 


scattered  also  over  the  inner  surface  of  the  blade.  These 
glands  excrete  a  clear,  sticky  fluid,  which  hangs  to  them  in 
drops  like  dew-drops.     If  a  small  insect  becomes  entangled 


Fig.  151.    Plants  of  Dioncea,  showing  the  rosette  habit  of  the  leaves  with  terminal 
traps,  and  the  erect  flowering  stem.— After  Kerner. 


in  the  sticky  drop,  the  hair  begins  to  curve  inward,  and 
presently  presses  its  victim  down  upon  the  surface  of  the 
blade.  In  the  case  of  larger  insects,  several  of  the  marginal 
hairs  may  join  together  in  holding  it,  or  the  whole  blade 
may  become  more  or  less  rolled  inward. 


THE   NUTRITION   OF   PLANTS. 


161 


(3)  Dioncea. — This  is  one  of  the  most  famous  and  re- 
markable of  ily-catching  plants  (see  Fig.  151).  It  is  found 
in  sandy  swamps  near  Wilmington,  North  Carolina.  The 
leaf  blade  is  constructed  like  a  steel  trap,  the  two  halves 
snapping  together,  and  the  marginal  bristles  interlocking 
like  the  teeth  of  a  trap  (see  Fig.  152).  A  few  sensitive 
hairs,   like  feelers,    arc 

developed   on   the    leaf  ^dk\Ww'k 

surface,  and  when  one 
of  these  is  touched  by 
a  small  flying  or  hover- 
ing insect,  the  trap 
snaps  shut  and  the  in- 
sect is  caught.  Only 
after  digestion  does  the 
trap  open  again. 

There  are  certain 
green  plants,  not  called 
carnivorous  plants, 
which  show  the  same 
general  habit  of  sup- 
plementing their  food 
supply,  and  so  reduc- 
ing the  necessity  of 
food  manufacture. 
The  mistletoe  is  a 
green  plant,  growing 
upon  certain  trees,  from 

which  it  obtains  some  food,  supplementing  that  which  it 
is  able  to  manufacture. 


Fig.  152.  Three  leaves  of  Dioticea,  showing 
the  details  of  the  trap  in  the  leaves  to  right 
and  left,  and  the  central  trap  in  the  act  of 
capturing  an  insect. 


CHAPTER  XL 

PLANT  ASSOCIATIONS:  ECOLOGICAL.  FACTORS. 

120.  Definition  of  a  plant  association. — From  the  previous 
chapters  it  has  been  learned  that  every  complex  plant  is  a 
combination  of  organs,  and  that  each  organ  is  related  in 
some  special  way  to  its  environment.  It  follows,  there- 
fore, that  the  whole  plant,  made  up  of  organs,  holds  a  very 
complex  relation  with  its  environment.  The  stem  demands 
certain  things,  the  root  other  things,  and  the  leaves  still 
others.  To  satisfy  all  of  these  demands,  so  far  as  possible, 
the  whole  plant  is  delicately  adjusted. 

The  earth's  surface  presents  very  diverse  conditions  in 
reference  to  plant  life,  and  as  plants  are  grouped  according 
to  these  conditions,  this  leads  to  definite  associations  of 
plants,  those  adapted  to  the  same  general  conditions  being 
apt  to  live  together.  Such  an  assemblage  of  plants  living 
together  in  similar  conditions  is  &  plant  association,  the  con- 
ditions forbidding  other  plants.  It  must  not  be  understood 
that  all  plants  affecting  the  same  conditions  will  be  found  liv- 
ing together.  For  example,  a  meadow  of  a  certain  type  will 
not  contain  all  the  kinds  of  grasses  associated  with  that  type. 
Certain  grasses  will  be  found  in  one  meadow,  and  other 
grasses  will  be  found  in  other  meadows  of  the  same  type. 

The  rivalry  of  closely  related  plants  living  in  the  same 
association  is  apt  to  be  intense,  on  account  of  their  similar 
demands,  and  unrelated  plants  are  able  to  live  together  with 
the  least  rivalry.  A  plant  association,  therefore,  may  con- 
tain a  wide  representation  of  the  plant  kingdom,  from 
plants  of  low  rank  to  those  of  high  rank. 
162 


PLANT  ASSOCIATIONS:    ECOLOGICAL  FACTORS.         163 

Before  considering  some  of  the  common  associations,  it 
is  necessary  to  note  some  of  the  conditions  that  deter- 
mine plant  associations.  Those  things  in  the  environment 
of  the  plant  which  influence  the  organization  of  an  associa- 
tion are  known  as  ecological  factors. 

121.  Water. — Water  is  certainly  one  of  the  most  im- 
portant conditions  in  the  environment  of  a  plant,  and  has 
great  influence  in  determining  the  organization  of  associa- 
tions. If  all  plants  are  considered,  it  will  be  noted  that  the 
amount  of  water  to  which  they  are  exposed  is  exceedingly 
variable.  At  one  extreme  are  those  plants  which  are  com- 
pletely submerged ;  at  the  other  extreme  are  those  plants 
of  arid  regions  which  can  obtain  very  little  water ;  and  be- 
tween these  extremes  there  is  every  gradation  in  the  amount 
of  available  water.  Among  the  most  striking  adaptations 
of  plants  are  those  for  living  in  the  presence  of  a  great 
amount  of  water,  and  those  for  guarding  against  its  lack. 

One  of  the  first  things  to  consider  in  connection  with 
any  plant  association  is  the  amount  of  water  supply.  It  is 
not  merely  a  question  of  its  total  annual  amount,  but  of  its 
distribution  through  the  year.  Is  it  supplied  somewhat 
uniformly,  or  is  there  alternating  flood  and  drought  ?  The 
nature  of  the  water  supply  is  also  important.  Are  there 
surface  channels  or  subterranean  channels,  or  does  the 
whole  supply  come  in  the  form  of  rain  and  snow  which 
fall  upon  the  area? 

Another  important  fact  to  consider  in  connection  with 
the  water  supply  has  to  do  with  the  structure  of  the  soil. 
There  is  what  may  be  called  a  water  level  in  soils,  and  it  is 
important  to  note  the  depth  of  this  level  beneath  the  sur- 
face. In  some  coils  it  is  very  near  the  surface ;  in  others, 
such  as  sandy  soils,  it  may  be  some  distance  beneath  the 
surface. 

Xot  only  do  the  amount  of  water  and  the  depth  of  the 
water  level  help  to  determine  plant  associations,  but  also  the 
substances  that  the  water  contains.  Two  areas  may  have 
12 


164  PLANT   RELATIONS. 

the  same  amount  of  water  and  the  same  water  level,  but  if 
the  substances  dissolved  in  the  water  differ  in  certain  par- 
ticulars, two  entirely  distinct  associations  may  result. 

122.  Heat. — The  general  temperature  of  an  area  is  im- 
portant to  consider,  but  it  is  evident  that  differences  of 
temperature  are  not  so  local  as  differences  in  the  water  sup- 
ply, and  therefore  this  factor  is  not  so  important  in  the 
organization  of  the  plant  associations  of  any  given  neigh- 
borhood as  is  the  water  factor.  Even  in  the  distribution 
of  plants  over  the  surface  of  the  earth,  however,  the  water 
factor  is  probably  more  important  than  the  heat  factor.  The 
range  of  temperature  which  the  plant  kingdom,  as  a  whole, 
can  endure  during  active  work  may  be  stated  in  a  general 
way  as  from  0°  to  50°  0. ;  that  is,  from  the  freezing  point 
of  water  to  122°  Fahr.  There  are  certain  plants  that  can 
work  at  higher  temperatures,  notably  certain  algas  growing 
in  hot  springs,  but  they  may  be  regarded  as  exceptions.  It 
must  be  remembered  that  the  range  of  temperature  given 
is  for  plants  actively  at  work,  and  does  not  include  the  tem- 
perature which  many  plants  are  able  to  endure  in  a  specially 
protected  but  very  inactive  condition.  For  example,  many 
plants  of  the  temperate  regions  endure  a  winter  tempera- 
ture which  is  frequently  lower  than  the  freezing  point  of 
water,  but  it  is  a  question  of  endurance  and  not  of  work 

It  must  not  be  supposed  that  all  plants  can  work  equally 
well  throughout  the  whole  range  of  temperature  given,  for 
they  differ  widely  in  this  regard.  Tropical  plants,  for  in- 
stance, accustomed  to  a  certain  limited  range  of  high  tem- 
perature, cannot  work  continuously  at  the  lower  tempera- 
tures. For  each  kind  of  plant  there  is  what  may  be  called 
a  zero  point,  below  which  it  is  not  in  the  habit  of  working. 

While  it  is  important  to  note  the  general  temperature 
of  an  area  throughout  the  year,  it  is  also  necessary  to  note 
its  distribution.  Two  regions  may  have  presumably  the 
same  amount  of  heat  through  the  year,  but  if  in  the  one  case 
it  is  uniformly  distributed,  and  in  the  other  great  extremes 


PLANT   ASSOCIATIONS:    ECOLOGICAL  FACTORS.  165 

of  temperature  occur,  the  same  plants  will  not  be  found  in 
both.  It  is,  perhaps,  most  important  to  note  the  tempera- 
ture during  certain  critical  periods  in  the  life  of  plants, 
such  as  the  flowering  period  of  seed-plants. 

Although  the  temperature  problem  may  be  compara- 
tively uniform  over  any  given  area,  the  effect  of  it  may  be 
noted  in  the  succession  of  plants  through  the  growing  sea- 
son. In  our  temperate  regions  the  spring  plants  and  summer 
plants  and  autumn  plants  differ  decidedly  from  one  another. 
It  is  evident  that  the  spring  plants  can  endure  greater 
cold  than  the  summer  plants,  and  the  succession  of  flowers 
will  indicate  somewhat  these  relations  of  temperature. 

It  should  be  remarked,  also,  that  not  only  is  the  tem- 
perature of  the  air  to  be  noted,  but  also  that  of  the  soil. 
These  two  temperatures  may  differ  by  several  degrees,  and 
the  soil  temperature  especially  affects  root  activity,  and 
hence  is  a  very  important  factor  to  discover. 

At  this  point  it  is  possible  to  call  attention  to  the  effect 
of  the  combination  of  ecological  factors.  For  instance,  in 
reference  to  the  occurrence  of  plants  in  any  association,  the 
water  factor  and  the  heat  factor  cannot  be  considered  each 
by  itself,  but  must  be  taken  in  combination.  For  example, 
if  in  a  given  area  there  is  a  combination  of  maximum  heat 
and  minimum  water,  the  result  will  be  a  desert,  and  only 
certain  specially  adapted  plants  can  exist.  It  is  evident 
that  the  great  heat  increases  the  transpiration,  and  trans- 
piration when  the  supply  of  water  is  very  meager  is  pecu- 
liarly dangerous.  Plants  which  exist  in  such  conditions, 
therefore,  must  be  specially  adapted  for  controlling  tran- 
spiration. On  the  other  hand,  if  in  any  area  the  combina- 
tion is  maximum  heat  and  maximum  water,  the  result  will 
be  the  most  luxuriant  vegetation  on  the  earth,  such  as 
grows  in  the  rainy  tropics.  It  is  evident  that  the  possible 
combinations  of  the  water  and  heat  factors  may  be  very 
numerous,  and  that  it  is  such  combinations  that  chiefly 
affect  plant  associations. 


166  PLANT  RELATIONS. 

123.  Soil. — The  soil  factor  is  not  merely  important  to 
consider  in  connection  with  those  plants  directly  related 
to  the  soil,  but  is  a  factor  for  all  plants,  as  it  determines 
the  substances  which  the  water  contains.  There  are  two 
things  to  be  considered  in  connection  with  the  soil,  namely, 
its  chemical  composition  and  its  physical  properties.  Per- 
haps the  physical  properties  arc  more  important  from  the 
standpoint  of  soil-related  plants  than  the  chemical  com- 
position, although  both  the  chemical  and  physical  nature 
of  the  soil  are  so  bound  up  together  that  they  need  not  be 
considered  separately  here.  The  physical  properties  of  the 
soil,  which  are  important  to  plants,  are  chiefly  those  which 
relate  to  the  water  supply.  It  is  always  important  to  de- 
termine how  receptive  a  soil  is.  Does  it  take  in  water 
easily  or  not  ?  It  is  also  necessary  to  determine  how  re- 
tentive it  is ;  it  may  receive  water  readily,  but  it  may  not 
retain  it. 

For  convenience  in  ordinary  field  work  with  plants, 
soils  may  be  divided  roughly  into  six  classes  :  (1)  rock, 
which  means  solid  uncrumbled  rock,  upon  which  certain 
plants  are  able  to  grow  ;  (2)  sand,  which  has  small  water 
capacity,  that  is,  it  may  receive  water  readily  enough,  but 
does  not  retain  it ;  (3)  lime  soil ;  (4)  clay,  which  has  great 
water  capacity  ;  (5)  humus,  which  is  rich  in  the  products 
of  plant  and  animal  decay  ;  ((5)  salt  soil,  in  which  the  water 
contains  certain  salts,  and  is  generally  spoken  of  as  alka- 
line. These  divisions  in  a  rough  way  indicate  both  the 
structure  of  the  soil  and  its  chemical  composition.  Not 
only  should  the  kinds  of  soil  on  an  area  be  determined, 
but  their  depth  is  an  important  consideration.  It  is 
very  common  to  find  one  of  these  soils  overlying  another 
one,  and  this  relation  between  the  two  will  have  a  very 
important  effect.  For  instance,  if  a  sand  soil  is  found 
lying  over  a  clay  soil,  the  result  will  be  that  the  sand  soil 
will  retain  far  more  water  than  it  would  alone.  If  a  humus 
soil  in  one  area  overlies  a  sand  soil,  and  in  another  area 


PLANT  ASSOCIATIONS:   ECOLOGICAL  tfAOTOKS.         1(J7 

overlies  a  clay  soil,  the  humus  will  differ  very  much  in  the 
two  cases  in  reference  to  water. 

The  soil  cover  should  also  be  considered.  The  common 
soil  covers  are  snow,  fallen  leaves,  and  living  plants.  It 
will  be  noticed  that  all  these  covers  tend  to  diminish  the 
loss  of  heat  from  the  soil,  as  Avell  as  the  access  of  heat  to 
the  soil.  In  other  words,  a  good  soil  cover  will  very  much 
diminish  the  extremes  of  temperature.  All  this  tends  to 
increase  the  retention  of  water. 

124.  Light. — It  is  known  that  light  is  essential  for  the 
peculiar  work  of  green  plants.  However,  all  green  plants 
cannot  have  an  equal  amount  of  light,  and  some  have 
learned  to  live  with  a  less  amount  than  others.  While 
no  sharp  line  can  be  drawn  between  green  plants  which 
use  intense  light,  and  those  which  use  less  intense  light, 
we  still  recognize  in  a  general  way  what  are  called  light 
plants  and  shade  plants.  We  know  that  certain  plants 
are  chiefly  found  in  situations  where  they  can  be  exposed 
freely  to  light,  and  that  other  plants,  as  a  rule,  are  found 
in  shady  situations. 

Starting  with  this  idea,  we  find  that  plants  grow  in 
strata.  In  a  forest  association,  for  example,  the  tall  trees 
represent  the  highest  stratum ;  below  this  there  may  be  a 
stratum  of  shrubs,  then  tall  herbs,  then  low  herbs,  then 
forms  like  mosses  and  lichens  growing  close  to  the  ground. 
In  any  plant  association  it  is  important  to  note  the  num- 
ber of  these  strata.  It  may  be  that  the  highest  stratum 
shades  so  densely  that  many  of  the  other  strata  are  not 
represented  at  all.  An  illustration  of  this  can  be  obtained 
from  a  dense  beech  forest. 

125.  Wind. — It  is  generally  known  that  wind  has  a  dry- 
ing effect,  and,  therefore,  it  increases  the  transpiration  of 
plants  and  tends  to  impoverish  them  in  water.  This  fac- 
tor is  especially  conspicuous  in  regions  where  there  are  pre- 
vailing winds,  such  as  near  the  seacoast,  around  the  great 
lakes,  and  on  the  prairies  and  plains.     In  all  such  regions 


168  PLANT   RELATIONS. 

the  plants  have  been  compelled  to  adapt  themselves  to  this 
loss  of  water ;  and  in  some  regions  the  prevailing  winds 
are  so  constant  and  violent  that  the  force  of  the  wind  itself 
has  influenced  the  appearance  of  the  vegetation,  giving 
what  is  called  a  characteristic  physiognomy  to  the  area. 

These  five  factors  have  been  selected  from  a  much 
larger  number  that  might  be  enumerated,  but  they  may 
be  regarded  as  among  the  most  important  ones.  It  will  be 
noticed  that  these  factors  may  be  combined  in  all  sorts 
of  ways,  so  that  an  almost  endless  series  of  combinations 
seems  to  be  possible.  This  will  give  some  idea  as  to  the 
possible  number  of  plant  associations,  for  they  may  be  as 
numerous  as  are  the  combinations  of  these  factors. 

126.  The  great  groups  of  associations. — It  is  possible  to 
reduce  the  very  numerous  associations  to  three  or  four 
great  groups.  For  convenience,  the  water  factor  is  chiefly 
used  for  this  classification.  It  results  in  a  convenient 
classification,  but  one  that  is  certainly  more  or  less  arti- 
ficial. The  selection  of  any  one  factor  from  among  the 
many  for  the  purpose  of  classification  never  results  in  a 
very  natural  classification  when  the  combination  of  factors 
determines  the  group.  However,  for  general  purposes,  the 
usual  classification  on  the  basis  of  water  supply  will  be 
used.  On  this  basis  there  are  three  great  groups  of  asso- 
ciations, as  follows : 

(1)  Hydrophytes. — The  name  means  "  water  plants,"  and 
suggests  that  such  associations  are  at  that  extreme  of  the 
water  supply  where  it  is  very  abundant.  Such  plants  may 
grow  in  the  water,  or  in  very  wet  soil,  but  in  any  event 
they  are  exposed  to  a  large  amount  of  water. 

(2)  Xerophytes. — The  name  means  "  drouth  plants,"  and 
suggests  the  other  extreme  of  the  water  supply.  True 
xerophytes  are  exposed  to  dry  soil  and  dry  atmosphere. 

(3)  Mesophytes. — Between  the  two  extremes  of  the 
water  supply  there  is  a  great  middle  region  of  medium 
water  supply,  and   plants  that   occupy  it  are  known   as 


PLANT   ASSOCIATIONS:    ECOLOGICAL  FACTORS.         169 

mesophytes,  the  plants  of  medium  conditions.  It  is  evi- 
dent that  mesophytes  gradually  pass  into  hydrophytes  on 
the  one  side,  and  into  xerophytes  on  the  other ;  but  it  is 
also  evident  that  mesophyte  associations  have  the  greatest 
range  of  water  supply,  extending  from  a  large  amount  of 
water  to  a  very  small  amount. 

It  should  be  understood  that  these  three  groups  of 
associations,  which  are  distinguished  from  one  another  by 
the  amount  of  the  water  supply,  are  artificial  groups  rather 
than  natural  ones,  for  they  bring  together  unrelated  asso- 
ciations, and  often  separate  those  that  are  closely  related. 
For  example,  a  swampy  meadow  is  put  among  hydrophyte 
associations  by  this  classification  ;  and  it  may  shade  into  an 
ordinary  meadow,  which  belongs  among  the  mesophytes. 
Probably  the  largest  fact  that  may  be  used  in  grouping 
plant  associations  is  that  certain  associations  are  so  situ- 
ated that  they  seek  for  the  most  part  to  reduce  transpira- 
tion, and  that  others  are  so  situated  that  they  seek  for  the 
most  part  to  increase  transpiration. 

However,  the  factors  that  determine  associations  are 
so  numerous  that  they  cannot  be  presented  in  an  elementary 
book,  and  the  simpler  artificial  grouping  given  above  will 
serve  to  introduce  the  associations  to  observation. 


CHAPTER  XII. 

HYDROPHYTE    ASSOCIATIONS. 

127.  General  character. — Hydrophytes  are  related  to 
abundant  water,  either  throughout  their  whole  structure 
or  in  part  of  their  structure.  It  is  a  well-known  fact  that 
hydrophytes  are  among  the  most  cosmopolitan  of  plants, 
and  hydrophyte  associations  in  one  part  of  the  world  look 
very  much  like  hydrophyte  associations  in  any  other 
region.  It  is  probable  that  the  abundant  water  makes  the 
conditions  more  uniform. 

It  is  evident  that  for  those  plants,  or  plant  parts,  which 
are  submerged,  the  water  affects  the  heat  factor  by  dimin- 
ishing the  extremes.  It  also  affects  the  light  factor,  in  so 
far  as  the  light  must  pass  through  the  water  to  reach  the 
chlorophyll-containing  parts,  as  light  is  diminished  in  in- 
tensity by  passing  through  the  water.  Before  considering 
a  few  hydrophyte  associations,  it  is  necessary  to  note  the 
prominent  hydrophyte  adaptations. 

128.  Adaptations. — In  order  that  the  illustration  may  be 
as  simple  as  possible,  a  complex  plant  completely  exposed 
to  water  is  selected,  for  it  is  evident  that  the  relations  of  a 
swamp  plant,  with  its  roots  in  water  and  its  stem  and  leaves 
exposed  to  air,  are  complicated.  A  number  of  adaptations 
may  be  noted  in  connection  Avith  the  submerged  or  floating 
plant. 

(1)  Thin-walled  epidermis. — In  the  case  of  the  soil-re- 
lated plants,  the  water  supply  comes  mainly  from  the  soil, 
and  the  root  system  is  constructed  to  absorb  it.  In  the 
case  of  the  water  plant  under  consideration,  however,  the 


HYDROPHYTE   ASSOCIATIONS. 


171 


whole  plant  body  is  exposed  to  the  water  supply,  and  there- 
fore absorption  may  take  place  through  the  whole  surface 
rather  than  at  any  particular  region  such  as  the  root.  In 
order  that  this  may  be  done,  however,  it  is  necessary  for 
the  epidermis  to  have  thin  walls,  which  is  usually  not  the 
case  in  epidermis  exposed 
to  the  air,  where  a  certain 
amount  of  protection  is 
needed  in  the  way  of 
thickening. 

(2)  Roots  much  reduced 
or  wanting. — It  must  be 
evident  that  if  water  is 
being  absorbed  by  the 
whole  free  surface  of  the 
plant,  there  is  not  so 
much  need  for  a  special 
root  region  for  absorp- 
tion. Therefore,  in  such 
water  plants  the  root  sys- 
tem may  be  much  re- 
duced, or  may  even  disap- 
pear entirely.  It  is  often 
retained,  however,  to  act 
as  a  holdfast,  rather  than 
as  an  absorbent  organ,  for 
most  water  plants  anchor 
themselves  to  some  sup- 
port. 

(3)  Reduction  of  water-conducting  tissues. — In  the  ordi- 
nary soil-related  plants,  not  only  is  an  absorbing  root  sys- 
tem necessary,  but  also  a  conducting  system,  to  carry  the 
water  absorbed  from  the  roots  to  the  leaves  and  elsewhere. 
It  has  already  been  noted  that  this  conducting  system  takes 
the  form  of  woody  strands.  It  is  evident  that  if  water 
is  being  absorbed  by  the  whole  surface  of  the  plant,  the 


Fig.  153.  Fragment  of  a  common  seaweed 
(Fitcus),  showing  the  body  with  forking 
branching  and  bladder-like  air  cavities.— 
After  Lueks^en. 


172 


PLANT   RELATIONS. 


work  of  conduction  is  not  so  extensive  or  definite,  and 
therefore  in  such  water  plants  the  woody  bundles  are  not 
so  prominently  developed  as  in  land  plants. 

(4)  Reduction   of  mechanical  tissues. — In  the   case   of 
ordinary  land  plants,  certain  firm  tissues  are  developed  so 


Fig.  154.    Gulfweed  (jSargassum),  showing  the  thallns  differentiated  into  stem-like  and 
leaf -like  portions,  and  also  the  bladder-like  floats.-  Alter  Bennktt  and  Mhkkay. 


that  the  plant  may  maintain  its  form.  These  supporting 
tissues  reach  their  culmination  in  such  forms  us  trees, 
where  massive  bodies  are  able  to  stand  upright.  It  is  evi- 
dent that  in  the  water  there  is  no  such  need  for  rigid  sup- 
porting tissues,  as  the  buoyant  power  of  water  helps  to 
support  the  plant.     This  fact  may  be  illustrated  by  taking 


HYDKOPHYTE   ASSOCIATIONS. 


173 


out  of  water  submerged  plants  which  seem  to  be  upright, 
with  all  their  parts  properly  spread  out.    When  removed  they 
collapse,  not  being  able  to  support  themselves  in  any  way. 
(5)  Development  of  air  cavities. — The  presence  of  air  in 
the  bodies  of  water  plants  is  necessary  for  two  reasons:  (1), 


,f    .-   % 


Fig.  155.  Bladderwort,  showing  the  numerous  bladders  which  float  the  plant,  the 
finely  divided  water  leaves,  and  the  erect  flowering  stems.  The  bladders  are  also 
effective  "insect  traps,"  Utricalaria  being  one  of  the  "carnivorous  plants." 
— After  Kerner. 


to  aerate  the  plant ;  (2),  to  increase  its  buoyancy.  In  most 
complex  water  plants  there  must  be  some  arrangement  for 
the  distribution  of  air  containing  oxygen.  This  usually 
takes  the  form  of  air  chambers  and  passageways  in  the 
body  of  the  plant  (see  Figs.  87,  88,  89,  156).  Of  course 
such  air  chambers  increase  the  buoyancy  of  the  body. 
Sometimes,  however,  a  special  buoyancy  is  provided  for 
by  the  development  of  regular  floats,  which  are  bladder- 


174  PLANT   RELATIONS. 

like  bodies  (see  Figs.  153,  154).  These  floats  are  very  com- 
mon  among  certain  of  the  seaweeds,  and  are  fonnd  among 
higher  plants,  as  the  utricularias  or  bladderworts,  which 
have  received  their  name  from  the  numerous  bladders 
developed  in  connection  with  their  bodies  (see  Fig.  155). 

129.  The  two  groups  of  associations. — The  hydrophyte 
associations  may  be  put  into  two  great  divisions.  True 
h  i/drophytes  are  those  in  which  the  contents  and  tempera- 
ture of  the  water  are  favorable  to  plant  activity;  while 
xerophytic  hydrophytes  are  those  in  which  the  contents 
and  temperature  of  the  water  are  unfavorable  to  plant 
activity,  and  the  structures  of  the  plants  are  adapted  to 
reduce  transpiration,  resembling  in  this  feature  the  struc- 
tures displayed  by  the  true  xerophytes  (see  §155). 


I.  True  hydrophytes. 
A.   Free-swimming  associations. 

130.  Definition. — In  these  associations  there  is  the  largest 
exposure  to  water,  and  no  relation  at  all  to  the  nutrient  or 
mechanical  support  of  the  soil,  the  plants  being  completely 
supported  by  the  water.  They  may  be  either  submerged 
or  floating,  and  they  are  free  to  move  either  by  locomotion 
or  by  water  currents.  Two  prominent  associations  are 
selected  as  types. 

131.  The  plankton. — This  term  is  used  to  designate  the 
minute  organisms,  both  plants  and  animals,  that  are  found 
in  the  water.  The  plankton  is  composed  of  individuals 
invisible  to  the  naked  eye,  but  taken  together  they  repre- 
sent an  enormous  organic  mass.  The  plankton  associa- 
tions are  especially  well  represented  in  the  colder  oceanic 
waters,  but  they  are  not  absent  from  any  waters.  Among 
the  most  prominent  plants  in  these  associations  are  the 
diatoms.  Diatoms  are  minute  plants  of  various  forms,  and 
all  have  a  wall  very  full  of  silica.     This  makes  their  bodies 


HYDROPHYTE   ASSOCIATIONS. 


175 


extremely  enduring,  and  therefore  diatoms  are  often  found 
in  great  deposits  in  the  rocks,  in  some  cases  forming  the 
whole  mass  of  rock.  Associated  with  the  diatoms  are 
numerous  other  plant  and  animal  forms. 

132.  Pond  associations. — The  word  pond  is  used  to  indi- 
cate stagnant  or  slow-moving  waters.  In  such  waters  free- 
swimming  plants  of  all  groups  are  associated.  Of  course 
the  alga?  arc  well  represented,  but  even  the  highest  plants 
are  repre- 
sented by  the 
duckweeds, 
which  are  very 
commonl  y 
seen  in  the 
form  of  small 
green  disks 
floating  on  the 
surface  of  the 
water,  which 
they  frequent- 
ly cover  with 
great   masses 

(see  Fig.  150).  It  should  be  observed  that  the  floating  and 
submerged  positions  result  in  a  difference  in  light-relations. 
The  floating  forms  may  be  regarded  as  light  forms,  being 
exposed  to  the  greatest  amount  of  light.  The  submerged 
forms  are  shade  plants,  and  the  shading  becomes  greater 
as  the  depth  of  the  water  is  greater.  It  must  not  be  sup- 
posed that  submerged  plants  can  live  at  any  depth,  for 
soon  a  limit  is  reached,  beyond  which  the  light  is  not 
intense  enough  to  enable  plants  to  work. 

It  has  been  noticed  that  this  complete  water  habit  has 
affected  plants  in  many  ways.  For  instance,  the  duck- 
weeds are  related  to  land  plants  with  root,  stem,  and  leaves, 
but  they  have  lost  the  distinction  between  stem  and  leaf, 
and  the  body  is  merely  a  flat  leaf-like  disk  floating  upon 


Fig.  156.  A  section  through  the  body  of  a  duckweed  (Lemnd), 
showing  the  air  spaces  (a)  which  make  it  buoyant,  the 
origin  (?■)  of  the  simple  dangling  root,  and  the  pockets 
(s  and  I)  from  which  new  plants  bud  out,  and  in  which 
flowers  are  developed. 


176  PLANT   RELATIONS. 

the  water,  with  a  few  roots  dangling  from  the  under  side, 
or  with  no  roots  at  all  (see  Fig.  156).  This  same  duck- 
weed also  shows  some  interesting  modifications  in  its  hab- 
its of  reproduction.  Although  related  to  plants  which  pro- 
duce flowers  and  make  seed,  the  duckweeds  have  almost 
lost  the  power  of  producing  flowers,  and  when  they  do 
produce  them,  seeds  are  very  seldom  formed.  In  other 
words,  the  ordinary  method  of  reproduction  employed 
by  flowering  plants  has  been  more  or  less  abandoned. 
Replacing  this  method  of  reproduction  is  a  great  power 
of  vegetative  propagation.  From  the  disk-like  body  of 
the  plant  other  disk-like  bodies  bud  out,  and  this  bud- 
ding continues  until  a  large  group  of  disks,  more  or 
less  connected  with  each  other,  may  be  formed.  These 
plants  also  form  what  are  known  as  winter  buds — well 
protected  bud-like  bodies  which  sink  to  the  bottom  of 
the  pond  when  the  floating  plants  are  destroyed,  and 
remain  protected  by  the  mucky  bottom  until  the  waters 
become  warm  again  in  the  next  growing  season. 

In  examining  the  pond  associations,  therefore,  attention 
should  be  paid  to  the  floating  forms  and  the  submerged 
forms,  and  also  to  the  varying  depths  of  the  latter.  It 
will  also  be  noted  that  the  leaves  of  floating  forms  are 
comparatively  broad,  while  those  of  submerged  forms  are 
narrow. 

B.   Anchored  associations. 

133.  Definition. — These  are  associations  fixed  to  the  soil 
but  with  submerged  or  floating  leaves.  In  this  case  there 
is  still  great  exposure  to  water,  but  there  is  also  a  definite 
soil  relation.  Two  prominent  associations  are  selected 
from  this  group  for  illustration. 

134.  Rock  associations. — The  term  rock  is  used  in  this 
connection  in  a  very  general  way,  meaning  simply  some  firm 
support  beneath  the  water ;  it  is  just  as  likely  to  be  a  stick 


HYDROPHYTE    ASSOCIATIONS. 


177 


as  a  stone.  Probably  the  most  prominent  group  of  plants 
affecting  these  conditions  are  algae,  both  fresh  water  and 
marine.     In  the  fresh  waters  very  many  of  the  algas  will  be 


'V 


r^ 


Pig.  157 


A  group  of  marine  seaweeds  (Laminarias).    Note  the  various  habits  of  the 
plant  body  and  the  root-like  holdfasts.— After  Rerner. 


found  anchored  to  some  support.  The  largest  display  of 
such  forms,  however,  is  found  among  the  marine  alga?, 
which  abound  along  all  seacoasts  (see  Fig.   157).     It  will 


Fig.  158.  A  natural,  but  nearly  overgrown,  lily  pond.  The  lily  pads  may  be  seen 
rising  more  or  leas  above  the  water  where  they  are  thickest.  The  forest  growth 
in  the  background  is  probably  a  tamarack  (larch)  swamp.  It  is  to  be  noticed  that 
as  the  lily  pond  loses  its  water  it  is  being  invaded  by  the  coarse  sedge  and  grass 
growth  of  a  swamp.  Between  the  lily  pond  and  the  forest  is  a  swamp-thicket. 
At  least  four  distinct  associations  are  represented  in  this  view.  A  fifth  is  prob- 
ably represented  in  the  form  of  plants  of  the  reed-swamp  type,  which  form  a 
transition  between  the  lily  pond  and  the  swamp-thicket. 


HYDROPHYTE   ASSOCIATIONS.  179 

be  noticed  that  the  habit  of  anchorage  demands  the 
development  of  special  organs  of  attachment,  which  usu- 
ally take  the  form  of  root-like  structures,  often  associated 
with  sucker-like  disks.  Associated  with  the  anchoring 
structures  is  often  a  development  of  floats,  which  is  es- 
pecially characteristic  of  seaweeds,  enabling  the  working 
body  to  float  freely  in  the  water  (see  Figs.  153,  154).  It  is 
evident  that  while  free-swimming  forms  may  be  suitable 
for  stagnant  waters,  anchored  forms  are  better  adapted  for 
moving  waters.  Therefore,  where  there  are  currents  of 
water,  or  wave  action,  the  anchored  forms  predominate. 
The  ability  to  live  in  moving  waters,  and  often  in  those 
that  become  violently  agitated,  has  its  advantage  to  the 
plant  in  the  more  rapidly  renewed  food  material.  In  such 
a  situation  free-swimming  forms  would  soon  be  stranded 
or  disposed  of  in  quieter  waters. 

In  the  case  of  the  marine  seaweeds  there  is  an  interest- 
ing relation  between  the  depth  of  the  water  and  the  color 
of  the  plants.  While  the  fresh  water  alga?  are  prevailingly 
green,  it  will  be  remembered  that  the  prevailing  colors  of 
the  alga?  of  the  seashore  are  brown  and  red.  The  brown 
often  passes  into  some  shade  of  yellow,  and  the  red  may 
merge  into  purple  or  violet,  but  in  general  the  two  types  of 
color  may  be  called  brown  and  red.  It  has  been  noticed 
that  the  brown  forms  are  found  at  less  depth  than  the  red 
forms,  so  that  in  a  general  way  there  are  two  zones  of  dis- 
tribution in  relation  to  depth,  the  red  zone  being  the  lower 
one  and  the  yellow  zone  the  upper.  Just  what  this  means 
in  the  economy  of  the  plants  is  not  clear,  but  it  has  been 
suggested  that  the  yellow  and  the  red  colors  assist  the 
chlorophyll  in  its  work,  which  is  more  or  less  interfered 
with  by  the  diminished  intensity  of  the  light  passing 
through  sea  water. 

135.  Loose  soil  associations. — This  phrase  is  used  merely 
to  contrast  with  rock  associations,  referring  to  the  fact  that 
the  anchorage  is  not  merely  for  mechanical  support,  but  that 
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HYDROPHYTE   ASSOCIATIONS.  181 

there  is  a  definite  relation  to  soil  in  which  roots  or  root-like 
structures  are  embedded.  Associations  of  this  type  contain 
the  greatest  variety  of  plants  of  all  ranks.  In  these  asso- 
ciations are  found  alga?,  mosses,  fern  plants,  pondweeds, 
water  lilies.,  etc.  (see  Figs.  158,  159,  160,  161).  Pondweeds 
and  water  lilies  may  be  taken  as  convenient  types  of  high 
grade  plants  which  grow  in  such  conditions. 

In  the  first  place,  it  will  be  noticed  that  they  are  in- 
clined to  social  growths,  great  numbers  of  individuals 
growing  together  and  forming  what  are  known  as  lily 
ponds  or  pondweed  beds,  although  in  the  small  lakes  of 
the  interior  where  pondweeds  abound  in  masses,  they  are 
more  commonly  known  as  "  pickerel  beds."  If  the  petiole 
of  a  lily  pad  be  traced  down  under  the  water,  it  will  be 
found  to  arise  from  an  intricate  mass  of  thick,  knotted 
stems,  So  extensively  do  these  stems  (rootstocks)  in  the 
mucky  bottom  branch  that  they  are  able  to  give  rise  to 
close  set  masses  of  leaves. 

Water  lilies  and  pondweeds  may  also  be  compared  to 
show  the  effect  of  the  floating  habit  in  contrast  with  the 
submerged  habit.  The  leaves  of  water  lilies  float  on  the 
surface,  and  therefore  are  broad  ;  and  being  exposed  to  light 
are  a  vivid  green,  indicating  the  abundant  development  of 
chlorophyll.  Many  of  the  pondweeds,  however,  are  com- 
pletely submerged.  As  one  floats  over  one  of  these  "  pick- 
erel beds,"  the  leafy  plants  may  be  seen  at  considerable 
depths,  and  have  a  pallid,  translucent  look.  It  will  be 
seen  that  in  these  cases  the  leaf  forms  are  narrow  rather 
than  broad,  often  being  ribbon-like,  or  in  some  submerged 
plants  even  cut  up  into  thread-like  forms.  It  is  evident 
that  such  narrow  leaf  forms  can  respond  more  easily  to 
water  movements  than  broad  forms.  The  pallid  look  of 
these  submerged  leaves  indicates  that  there  has  not  been 
an  abundant  development  of  chlorophyll.  Some  pondweeds, 
however,  have  both  types  of  leaves,  some  being  submerged 
and  others  floating.    In  these  cases  it  is  interesting  to  notice 


Fig  160  A  group  of  pondweeds.  The  stems  are  sustained  in  an  erect  position  by 
the  water,  and  the  narrow  leaves  are  exposed  to  a  light  whose  intensity  is  dimin- 
ished by  passing  through  the  water.— After  Keener, 


HYDKOPHYTE  ASSOCIATIONS.  183 

the  corresponding  change  of  form ;  on  the  same  individual 
the  submerged  leaves  are  very  narrow,  or  divided  into  very- 
narrow  lobes,  while  the  floating  ones  are  broad  (see  Fig. 
162).  The  relation  of  the  plant  to  the  water,  therefore,  has 
determined  the  leaf  form.  The  advantage  of  the  floating 
habit  of  leaves  is  not  merely  a  better  relation  to  light,  but 
the  carbon  dioxide  used  in  photosynthesis  and  the  oxygen 
used  in  respiration  may  be  obtained  freely  from  the  air, 
rather  than  from  the  water.  It  will  also  be  noticed  that 
these  water  plants  usually  send  their  flowers  to  the  surface, 
indicating  that  such  a  position  is  more  favorable  for  the 
work  of  the  flower  than  a  submerged  position.  Any  asso- 
ciation of  this  type  will  furnish  abundant  material  for 
observation,  and  it  is,  perhaps,  the  most  valuable  type  of 
association  for  study  that  has  been  mentioned  so  far. 

C.    Swamp  associations. 

136.  Definition. — In  swamp  associations  the  plants  are 
rooted  in  water,  or  in  soils  rich  in  water,  but  the  stems 
bearing  the  leaves  rise  above  the  surface.  Among  the 
hydrophytes,  swamp  plants  are  least  exposed  to  water,  and 
as  the  stem  and  its  leaves  are  exposed  to  the  air,  there  is 
no  such  reduction  of  the  root  system  and  of  conducting 
and  mechanical  tissues  as  in  the  other  hydrophytes.  Also 
the  epidermis  is  not  thin,  and  there  is  no  development  of 
floats  to  increase  the  buoyancy.  However,  the  root  must 
be  aerated,  and  hence  air  chambers  and  passageways  are 
abundant.  In  ordinary  cases  the  air  is  admitted  through 
openings  in  the  epidermis  of  the  stem  and  leaves,  and  so 
enters  the  air-passageways  that  are  continuous  to  the  roots. 
It  has  been  claimed  that  a  still  more  elaborate  arrangement 
for  root-aeration  exists  in  the  so-called  "  knees  "  of  cypress 
swamps,  which  are  special  growths  from  the  submerged 
root  system  and  rise  above  the  surface  of  the  water  (see 
Fig.  91).  It  has  been  shown  that  if  such  swamps  are 
flooded  above  the  level  of  the  knees  many  of  the  trees  are 


^zr 


Fig  161  Eel  grass  (Vallisneria),  a  common  pondweed  plant.  The  plants  are 
anchored  and  the  foliage  is  submerged.  The  carpel-bearing  flowers  are  carried  to 
the  surface  on  long  stalks  which  allow  a  variable  depth  of  water.  The  stamen- 
bearing  flowers  remain  submerged,  as  indicated  near  the  lower  left  corner,  the 
flowers  breaking  away  and  rising  to  the  surface,  where  they  float  and  effect  pollina- 
tion.—After  Kerneb. 


HYDKOPHYTE  ASSOCIATIONS.  185 

killed,  but  that  aeration  of  the  root  system  occurs  through 
the  knees  remains  to  be  proved. 

Another  habit  of  swamp  plants  is  called  turf-building, 
which  means  that  new  individuals  arise  from  older  ones, 
and  so  a  dense  mat  of  roots  and  rootstocks  is  formed.  Very 
prominent   among   these   turf-building   swamp  plants  are 


Fig.  162.  Two  leaves  of  a  water  buttercup,  showing  the  difference  in  the  forms  of 
submerged  ami  aerial  leaves  on  the  same  plant,  the  former  being  much  more 
finely  divided. — After  Strasburger. 

the  sedges.     Some  of  the  prominent  swamp  societies  may  be 
enumerated  as  follows  : 

137.  Reed  swamps. — The  reed-swamp  plants  arc  tall  wand- 
like forms,  which  grow  in  rather  deep,  still  water  (see  Fig. 
163).  Prominent  as  types  are  the  cat-tail  Hag.  bulrushes, 
and  reed  grasses.  Such  an  assemblage  of  forms  usually 
characterizes  the  shallow  margins  of  small  lakes  and  ponds. 
In  such  places  the  different  plants  are  apt  to  lie  arranged 
according  to  depth,  the  bulrushes  standing  in  the  deepest 
water,   and  behind  them  the  reed  grasses,  and   then  the 


186 


PLANT  RELATIONS. 


cat-tails.     This  regular  arrangement  in  zones  is  so  often 
interfered  with,  however,  that  it  is  not  always  evident. 

The  reed-swamp  associations  have  been  called  "  the  pio- 
neers of  land  vegetation,"  for  their  bodies  and  the  detritus 


Fig.  163.  A  reed  swamp,  fringing  the  low  shore  of  a  lake  or  a  sluggish  stream.  The 
plants  are  tall  and  wand-like,  and  all  are  monocotyls.  Three  types  are  prominent, 
the  reed  grasses  (the  tallest),  the  cat-tails  (at  the  right),  and  the  bulrushes  (a  group 
standing  out  in  deeper  water  near  the  middle  of  the  fringing  growth).  The  plant 
iu  the  foreground  at  the  extreme  right  is  the  arrow-leaf  (Sagittaria),  recognized 
by  its  characteristic  leaves.— After  Keener. 


make  the  water  more  and  more  shallow,  until  finally  the 
reed  plants  are  compelled  to  migrate  into  deeper  water 
(see  §108).  In  this  way  small  lakes  and  ponds  may  be 
completely  reclaimed,  and  become  converted  first  into 
ordinary  swamps,  and  finally  into  wet  meadows.    Instances 


HYDROPHYTE  ASSOCIATIONS.  187 

of  nearly  reclaimed  ponds  may  be  noticed,  where  bul- 
rushes, cat-tail  flags,  and  reed  grasses  still  occupy  certain 
wet  spots,  but  are  shut  off  from  further  migration.  The 
social  growth  of  these  plants,  brought  about  by  extensive 
root  stock  development,  is  especially  favorable  for  detain- 
ing detritus  and  building  a  land  surface. 

Keed-swamp  plants  also  have  in  general  a  tall  and  un- 
branched  habit  of  body.  They  may  be  bare  and  leafless, 
with  a  terminal  cluster  of  flowers,  as  in  the  bulrushes ;  or 
the  wand-like  stems  may  bear  long,  linear  leaves,  as  in  the 
cat-tails ;  or  the  stem  may  be  a  tall  stalk  with  two  rows  of 
narrow  leaves,  as  in  the  reed  grasses.  2so  more  character- 
istic group  of  forms  is  found  in  any  association.  Of  course, 
associated  with  these  forms  are  also  free  and  fixed  hydro- 
phytes, that  characterize  the  other  associations. 

138.  Swamps. — The  word  is  used  to  include  the  ordinary 
meadow-like  expanses  of  swampy  ground,  but  does  not  in- 
clude such  associations  as  peat  bogs.  There  is  less  water 
than  in  the  case  of  the  reed  swamps,  and  often  very  little 
standing  water.  One  of  the  peculiarities  of  the  swamp 
is  that  the  water  is  rich  in  available  soil  materials  used 
in  food  manufacture,  notably  the  nitrates  from  which 
nitrogen  is  obtained  for  proteid  manufacture.  In  such 
conditions,  therefore,  the  vegetation  is  dense,  and  the  soil 
is  black  with  the  humus  derived  from  the  decaying  plant 
bodies. 

Typical  swamps  border  the  reed  swamps  on  the  land 
side,  and  slowly  encroach  upon  them  as  the  reed  plants 
build  up  land.  Probably  the  most  characteristic  plant 
forms  of  the  swamp  association  are  the  sedges,  and  asso- 
ciated with  them  are  certain  coarse  grasses.  These  give 
the  meadow-like  aspect  to  the  swamp,  although  these 
grass-like  forms  are  very  coarse.  Along  with  the  domi- 
nant sedges  and  grasses  are  numerous  other  plants  adapted 
to  such  conditions,  such  as  some  of  the  buttercups.  It 
would  be  impracticable  to  give  a  list  of  swamp  plants,  as 


188  PLANT  RELATIONS. 

the  forms  associated  with  sedges  and  grasses  may  vary 
widely  in  different  associations  (Fig.  164). 

In  almost  all  swamps  there  is  a  lower  stratum  of  vege- 
tation than  that  formed  by  the  sedges.  This  lower  stratum 
is  made  of  certain  swamp  mosses,  which  grow  in  very  dense 
masses.  Towards  the  north,  where  the  temperature  con- 
ditions are  not  so  favorable  for  the  sedge  stratum,  it  may 
be  lacking  almost  entirely,  and  only  the  lower  moss  stratum 
left.  In  these  cases  the  swamp  becomes  little  more  than  a 
great  bed  of  moss,  and  it  is  in  such  conditions  that  peat 
may  be  formed. 

139.  Swamp-thickets. — Swamp-thickets  are  very  closely 
associated  with  swamps,  and  are  doubtless  derived  from 
them.  If  a  swamp,  with  its  sedge  stratum  and  moss 
stratum,  be  invaded  by  shrubs  or  low  trees,  it  becomes  a 
swamp-thicket.  It  will  be  noticed  that  these  shrubs  and 
trees  are  of  very  uniform  type,  being  mainly  willows,  alders, 
birches,  etc.  Such  willow  and  alder  thickets  are  very  com- 
mon in  high  latitudes. 


II.   Xerophytic  hydrophytes. 

A.   Fresh-water  associations. 

140.  Sphagnum-moors. — The  sphagnum-moor  is  a  very 
peculiar  type  of  swamp  association.  It  is  so  named  because 
the  common  bog  or  peat  moss,  known  as  sphagnum,  gives 
a  peculiar  stamp  to  the  whole  area.  Sphagnums  are  large, 
pale  mosses,  whose  lower  parts  die,  and  whose  upper  parts 
continue  to  live  and  put  out  new  branches,  so  that  a  dense 
turf  is  formed.  In  walking  over  such  a  bog  the  moss  turf 
seems  springy,  and  sometimes  trembles  so  as  to  suggest 
the  name  "  quaking  bog."  These  are  the  great  peat-form- 
ing bogs.  It  is  interesting  to  know  what  conditions  keep 
the  swamp  plants  out  of  the  sphagnum-moor.  The  plants 
of  the  sphagnum-moor  seem  to  be  entirely  different  from 


HYDROPHYTE   ASSOCIATIONS.  189 

those  of  the  swamp  association,  although  the  amount  of 
water  is  approximately  the  same.  Xot  only  are  the  plants 
different  in  the  sphagnum-moor,  but  they  are  not  so 
numerous,  and,  with  the  exception  of  the  moss,  do  not 
grow  so  densely.  Creeping  plants  are  common ;  certain 
kinds  of  sedges  and  grasses  are  found,  but  generally  not 
those  of  the  swamps ;  while  heaths  and  orchids  are  espe- 
cially abundant.  It  is  in  these  sphagnum-moors,  also,  that 
the  curious  forms  of  carnivorous  plants  are  developed,  among 
which  the  pitcher  plants,  droseras,  and  dionaeas  have  been 
described.  In  considering  this  strange  collection  of  forms, 
it  is  evident  that  there  must  be  some  peculiarity  in  the 
conditions  of  living.  Heaths  and  orchids  are  well-known 
mycorhiza  forms ;  the  carnivorous  plants  are  so  named 
because  they  capture  insects  to  supplement  their  food 
supply ;  while  the  peculiar  sphagnum  mosses  replace  the 
mosses  of  ordinary  swamps.  "What  causes  have  resulted  in 
an  association  of  such  marked  physiognomy  are  unknown, 
but  the  subject  is  attracting  much  attention. 

It  seems  evident  that  the  problem  is  one  of  absorption 
and  that  some  condition  is  interfering  with  this  important 
function.  One  conclusion,  based  upon  experimental  work, 
is  that  the  greater  coldness  of  the  bog  water  is  the  cause  of 
diminished  absorption,  for  the  difference  between  the  tem- 
perature of  bog  water  and  of  other  waters  is  quite  remark- 
able. Another  conclusion  is  that  certain  salts  dissolved  in 
the  bog  water  tend  to  check  the  power  of  absorption.  In 
any  event,  the  assemblage  of  bog  plants  is  made  up  of 
forms  that  have  learned  to  live  with  a  diminished  power  of 
absorption. 

It  is  usually  stated  that  the  water  of  the  sphagnum- 
moor  is  very  poor  in  the  food  materials  which  are  abundant 
in  the  water  of  swamps,  and  that  there  is  a  special  lack  of 
the  materials  which  are  used  in  the  manufacture  of  pro- 
teids.  If  this  is  true,  it  would  be  necessary  to  obtain  some 
proteid  material  already  formed,  and  this  might  account 


190  PLANT  RELATIONS. 

for  the  carnivorous  habit  and  for  the  sphagnum  mosses. 
Of  course  it  would  also  account  to  a  certain  extent  for  the 
exclusion  of  the  characteristic  swamp  plants.  It  is  a  well- 
known  fact  that  bodies  of  men  and  animals  that  have 
become  submerged  in  sphagnum-bogs  may  not  decay,  but 
have  been  found  preserved  after  a  very  long  period.  This 
will  also  indicate  why  such  bogs  are  especially  favorable  for 
peat  formation. 

These  two  associations,  therefore,  may  be  contrasted  as 
follows :  The  swamp  is  rich  in  plant  food,  and  is  character- 
ized chiefly  by  grassy  plants ;  the  sphagnum-moor  is  poor 
in  food  material  and  unfavorable  to  absorptiou,  and  is  char- 
acterized chiefly  by  sphagnum  moss.  It  will  be  noted  that 
peat  may  be  formed  in  connection  with  both,  but  in  the 
swamp  the  plant  forms  cannot  be  distinguished  in  the  peat, 
as  they  have  been  more  or  less  disorganized  through  decay, 
while  in  the  peat  of  the  sphagnum-moor  the  plant  forms 
are  well  preserved.  The  peat  of  the  swamp,  also,  yields  a 
great  amount  of  ash,  for  the  swamp  is  rich  in  soil  materials, 
while  the  peat  of  the  sphagnum-moor  yields  very  little  ash. 

141.  Swamp-forests.  —  It  was  noted  that  the  special 
types  of  shrub  or  tree  growth  associated  with  the  swamp 
conditions  are  willows,  alders,  birches,  etc.  In  the  same 
way  there  is  a  peculiar  tree  type  associated  with  the 
sphagnum-moor.  It  is  very  common  to  have  a  sphagnum 
area  occupied  by  trees,  and  the  area  becomes  a  swamp 
forest,  rather  than  a  sphagnum-moor.  The  chief  tree 
type  which  occupies  such  conditions  is  the  conifer  type, 
popularly  known  as  the  evergreens.  The  swamp  forests, 
therefore,  with  a  sphagnum-moor  foundation,  are  made  up 
of  larches,  certain  hemlocks  and  pines,  junipers,  etc.,  and 
towards  the  south  the  cypress  comes  in  (see  Fig.  1G5). 
The  larch  is  a  very  common  swamp  tree  of  the  northern 
regions,  where  such  an  area  is  commonly  called  a  "tama- 
rack swamp"  (see  Fig.  158).  The  larch  forests  are  apt  to 
be  in  the  form  of  small  patches,  while  the  larger  swamp 


192 


PLANT  RELATIONS. 


forests  are  made  of  dense  growths  of  hemlocks,  pines,  etc. 
In  the  densest  of  these  forests  the  shade  is  so  complete 
that  there  may  be  very  few  associated  plants  occurring  in 
strata  between  the  sphagnum  moss  and  the  trees.  In  the 
larch  forests,  however,  the  undergrowth  may  be  very  dense. 

B.  Salt-water  associations. 

142.  Mangrove  swamps. — This  is  certainly  the  most 
vigorous  of  the  salt-water  associations.  Mangrove  swamps 
occur  along  flat  tropical  seacoasts,  where  the  waters  are  quiet. 


m 


Fig.  166.    A  mangrove  forest  advancing  into  the  water.— After  Schtmper. 


The  mangrove  is  a  tree  of  curious  habit,  which  advances 
slowly  out  into  the  water  and  extends  back  landwards  as 
low  woods  or  thickets  (see  Figs.  166,  167).  The  whole 
surroundings  appear  forbidding,  for  the  water  is  sluggish 
and  mucky,  covered  with  scum,  rich  in  bacteria,  and  with 
bubbles  constantly  breaking  upon  the  surface  from  decay- 
ing matter  beneath  the  water.     The  mangrove  has  the  pe- 


14 


192b  PLANT  RELATIONS. 

culiarity  of  germinating  its  seeds  while  still  upon  the  tree, 
so  that  embryos  hang  from  the  trees,  and  then  drop  like 
plumb-bobs  into  the  muck  beneath,  where  they  stick  fast 
and  are  immediately  in  a  condition  to  establish  themselves. 
In  these  mangrove  swamps  the  species  are  few,  and  the 
adaptations  chiefly  in  the  way  of  developing  various  kinds 
of  holdfasts  for  anchoring  in  the  uncertain  soil,  and  also 
various  devices  for  carrying  air  to  the  submerged  parts. 

143.  Beach  marshes  and  meadows.— The  salt  marshes 
and  meadows  near  the  seacoast  are  very  well  known.  They 
lie  beyond  the  reach  of  ordinary  flood  tide,  but  the  waters 
are  brackish.  In  these  marshes  and  meadows  occur  certain 
characteristic  salt-water  grasses  and  sedges.  Such  forms 
being  the  dominant  type  give  the  general  appearance  of 
a  coarse  meadow.  Very  characteristic  of  such  associations 
are  also  certain  succulents,  such  as  samphire  {Salicornia), 
sea  blite  (Suceda),  saltwort  (Salsola),  etc.  In  fact,  this 
succulent  character  seems  to  be  a  direct  response  to  the 
saline  conditions.  The  difference  between  a  marsh  and 
meadow  is  simply  a  question  of  the  amount  of  water. 


CHAPTER  XIII. 

XEROPHYTE  ASSOCIATIONS. 

144.  General  character. — Strongly  contrasted  with  the 
hydrophytes  are  the  xerophytes,  which  are  adapted  to  dry 
air  and  soil.  The  xerophytic  conditions  may  be  regarded 
in  general  as  drouth  conditions.  It  is  not  necessary  for 
the  air  and  soil  to  be  dry  throughout  the  year  to  develop 
xerophytic  conditions.  These  conditions  may  be  put  under 
three  heads  :  (1)  possible  drouth,  in  which  a  season  of 
drouth  may  occur  at  irregular  intervals,  or  in  some  seasons 
may  not  occur  at  all ;  (2)  periodic  drouth,  in  which  there 
is  a  drouth  period  as  definite  as  the  winter  period  in  cer- 
tain regions  ;  (3)  perennial  drouth,  in  which  the  dry  con- 
ditions are  constant,  and  the  region  is  distinctly  an  arid 
or  desert  region. 

However  xerophytic  conditions  may  occur,  the  problem 
of  the  plant  is  always  one  of  water  supply,  and  many  strik- 
ing structures  have  been  developed  to  answer  it.  Plants 
in  such  conditions  must  provide,  therefore,  for  two  things : 
(1)  collection  and  retention  of  water,  and  (2)  prevention  of 
its  loss.  It  is  evident  that  in  these  drouth  conditions  the 
loss  of  water  through  transpiration  (see  §26)  tends  to  be 
much  increased.  This  tendency  in  the  presence  of  a  very 
meager  water  supply  is  a  menace  to  the  life  of  the  plant, 
for  it  is  impossible  to  stop  transpiration  entirely,  as  it 
must  take  place  so  long  as  the  plant  is  alive.  The  adapta- 
tions on  the  part  of  the  plant,  therefore,  are  directed 
towards  the  regulation  of  transpiration,  that  it  may  occur 


194  PLANT   KELATIONS. 

sufficiently  for  the  life-processes,  but  that  it  may  not  be 
wasteful  to  the  point  of  danger. 

The  regulation  of  transpiration  may  be  accomplished 
in  two  general  ways.  It  will  be  remembered  that  the 
amount  of  transpiration  holds  some  relation  to  the 
amount  of  leaf  exposure  or  exposure  of  green  tissue. 
Therefore,  if  the  amount  of  leaf  exposure  be  diminished, 
the  total  amount  of  transpiration  will  be  reduced.  Another 
general  way  for  regulating  transpiration  is  to  protect 
the  exposed  surface  in  some  way  so  that  the  water  does 
not  escape  so  easily.  In  a  word,  therefore,  the  general 
method  is  to  reduce  the  extent  of  exposed  surface  or  to 
protect  it.  It  must  be  understood  that  plants  do  not  differ 
from  each  other  in  adopting  one  or  the  other  of  these 
methods,  for  both  are  very  commonly  used  by  the  same 
plant. 

Adaptations. 

145.  Complete  desiccation. — Some  plants  have  a  very  re- 
markable power  of  completely  drying  up  during  the  drouth 
period,  and  then  reviving  upon  the  return  of  moisture. 
This  power  is  strikingly  illustrated  among  the  lichens  and 
mosses,  some  of  which  can  become  so  dry  that  they  may  be 
crumbled  into  powder,  but  revive  when  moisture  reaches 
them.  A  group  of  club  mosses,  popularly  known  as  "  res- 
urrection plants,"  illustrates  this  same  power.  The  dried 
up  nest-like  bodies  of  these  plants  are  common  in  the 
markets,  and  when  they  are  placed  in  a  bowl  of  water  they 
expand  and  may  renew  their  activity.  In  such  cases  it  can 
hardly  be  said  that  there  is  any  special  effort  on  the  part  of 
the  plant  to  resist  drouth,  for  it  seems  to  yield  completely 
to  the  dry  conditions  and  loses  its  moisture.  The  power 
of  reviving,  after  being  completely  dried  out,  is  an  offset, 
however,  for  protective  structures. 

146.  Periodic  reduction  of  surface. — In  regions  of  periodic 


XEKOPIIYTE   ASSOCIATIONS. 


195 


drouth  it  is  very  com- 
mon for  plants  to 
diminish  the  exposed 
surface  in  a  very  de- 
cided way.  In  such 
cases  there  is  what 
may  be  called  a  peri- 
odic surface  decrease. 
For  example,  annual 
plants  remarkably 
diminish  their  ex- 
posed surface  at  the 
period  of  drouth  by 
being  represented 
only  by  well-pro- 
tected seeds.  The 
whole  exposed  sur- 
face of  the  plant, 
root,  stem,  and  leaves, 
has  disappeared,  and 
the  seed  preserves  the 
plant  through  the 
drouth. 

Little  less  remark- 
able is  the  so-called 
geophilous  habit.  In ' 
this  case  the  whole  of 
the  plant  surface  ex- 
posed to  the  air  dis- 
appears, and  only 
underground  parts, 
such  as  bulbs,  tu- 
bers, etc.,  persist  (see 
Figs.  45,  46,  66,  67, 
68,  69,  70,  75,  144, 
168, 169).    At  the  re- 


Fig.  168.  The  bloodroot  (Sanguinaria),  showing 
the  subterranean  rootstock  sending  leaves  and 
flower  above  the  surface. — After  Atkinson. 


196 


PLANT  RELATIONS. 


Fig.   169.     The     spring 
beauty     ( Claytonia), 
showing  subterranean 
tuber-like  stem  sending  leaf  and  flower-bearing 
stem  above  the  surface.— After  Atkinson. 


turn  of  the  moist  season 
these  underground  parts 
develop  new  exposed 
surfaces.  In  such  cases 
it  may  be  said  that  at 
the  coming  of  the  drouth 
the  plant  seeks  a  sub- 
terranean retreat. 

A  little  less  decrease 
of    exposed    surface    is 
shown  by  the  deciduous 
habit.    It  is  known  that 
certain  trees  and  shrubs, 
whose     bodies     remain 
exposed  to  the  drouth, 
shed    their    leaves  and 
thus  very  greatly  reduce 
the  amount  of  exposure  : 
with  the  return  of  mois- 
ture, new  leaves  are  put 
forth.      It   will   be   re- 
marked, in  this  connec- 
tion,    that     the     same 
habits  serve  just  as  well 
to  bridge  over  a  period 
of  cold  as  a   period  of 
drouth,     and     perhaps 
they  are  more  familiar 
in  connection  with  the 
cold  period  than  in  con- 
nection with  the  drouth 
period. 

147.  Temporary  reduc- 
tion of  surface. — While 
the  habits  above  have  to 
do  with  regular  drouth 


XEKOPHYTE  ASSOCIATIONS.  197 

periods,  there  are  other  habits  by  which  a  temporary  re- 
duction of  surface  may  be  secured.  For  instance,  at  the 
approach  of  a  period  of  drouth,  it  is  very  easy  to  observe 
certain  leaves  rolling  up  in  various  ways.  As  a  leaf  be- 
comes rolled  up,  it  is  evident  that  its  exposed  surface  is 
reduced.  The  behavior  of  grass  leaves,  under  such  cir- 
cumstances, is  very  easily  noted.  A  comparison  of  the  grass 
blades  upon  a  well-watered  lawn  with  those  upon  a  dried-up 
lawn  will  show  that  in  the  former  case  the  leaves  are  flat, 
and  in  the  latter  more  or  less  rolled  up.  The  same  habit 
is  also  very  easily  observed  in  connection  with  the  larger- 
leaved  mosses,  which  are  very  apt  to  encounter  drouth 
periods. 

148.  Fixed  light  position. — In  general,  when  leaves  have 
reached  maturity,  they  are  unable  to  change  their  position 
in  reference  to  light,  having  obtained  what  is  known  as  a 
fixed  light  position.  During  the  growth  of  the  leaf,  how- 
ever, there  may  be  changes  in  direction  so  that  the  fixed 
light  position  will  depend  upon  the  light  direction  during 
growth.  The  position  finally  attained  is  an  expression  of 
the  attempt  to  secure  sufficient,  but  not  too  much  light 
(see  §13).  The  most  noteworthy  fixed  positions  of  leaves 
are  those  which  have  been  developed  in  intense  light. 
A  very  common  position  in  such  cases  is  the  profile  posi- 
tion, in  which  the  leaf  apex  or  margin  is  directed  upwards, 
and  the  two  surfaces  are  more  freely  exposed  to  the  morn- 
ing and  evening  rays — that  is,  the  rays  of  low  intensity — 
than  to  those  of  midday. 

Illustrations  of  leaves  with  one  edge  directed  upwards 
can  be  obtained  from  the  so-called  compass  plants.  Prob- 
ably most  common  among  these  are  the  rosin-weed  of  the 
prairie  region,  and  the  prickly  lettuce,  which  is  an  intro- 
duced plant  very  common  in  waste  ground  (see  Fig.  170). 
Such  plants  received  their  popular  name  from  the  fact  that 
many  of  the  leaves,  when  edgewise,  point  approximately 
north  and  south,  but  this  direction  is  very  indefinite.     It  is 


198 


PLANT  KELATIONS. 


evident  that  such  a  position  avoids  exposure  of  the  leaf 
surface  to  the  noon  rays,  but  obtains  for  these  same  sur- 
faces the  morning  and  evening  rays.  If  these  plants  are 
developed  in  the  shade,  the   "  compass"  habit  does  not 


Fig.  170.  Two  compass  plants.  The  two  figures  to  the  left  represent  the  same  plant 
{Silphium)  viewed  from  the  east  and  from  the  south.  The  two  figures  to  the  right 
represent  the  same  relative  positions  of  the  leaves  of  Lactuca. — After  Kerner. 

appear  (see  §15).  The  profile  position  is  :i  very  common 
one  for  the  leaves  of  Australian  plants,  a  fact  which  gives 
much  of  the  vegetation  a  peculiar  appearance.  All  these 
positions  are  serviceable  in  diminishing  the  loss  of  water, 
which  would  occur  with  exposure  to  more  intense  light. 
149.  Motile  leaves. — Although  in  most  plants  the  mature 


XEROPHYTE  ASSOCIATIONS. 


199 


leaves  are  in  a  fixed  position,  there  are  certain  ones  whose 
leaves  are  able  to  perform  movements  according  to  the  need. 
Mention  has  been  made  already  of  such  forms  as  Oxalis 
(see  §1-4),  whose  leaves  change  their  position  readily  in 
reference  to  light.  Motile  leaves  have  been  developed  most 
extensively  among  the  Leguminosce,  the  family  to  which 


Fig.  171.  Two  twigs  of  a  sensitive  plant.  The  one  to  the  left  shows  the  numerous 
small  leaflets  in  their  expanded  position  ;  the  one  to  the  right  shows  the  greatly 
reduced  surface,  the  leaflets  folded  together,  the  main  leaf  branches  having 
approached  one  another,  and  the  main  leaf-stalk  having  bent  sharply  downwards. 
—After  Strasburger. 


belong  peas,  etc.  In  this  family  are  the  so-called  "  sen- 
sitive plants,"  which  have  received  their  popular  name 
from  their  sensitive  response  to  light  as  well  as  to  other 
influences  (see  Fig.  171).  The  acacia  and  mimosa  forms 
are  the  most  notable  sensitive  plants,  and  tire  especially 
developed  in  arid  regions.  The  leaves  are  usually  very 
large,  but  are  so  much  branched  that  each  leaf  is  com- 
posed  of  very  numerous  small  leaflets.     Each  leaflet  has 


200 


PLANT  RELATIONS. 


the  power  of  independent  motion,  or  the  whole  leaf  may 
move.  If  there  is  danger  from  exposure  to  drouth,  some 
of  the  leaflets  will  be  observed  to  fold  together  ;  in  case 


Fig.  172.    A  heath  plant  (Erica),  showing  low,  bushy  growth  and  small  leaves. 

the  danger  is  prolonged,  more  leaflets  will  fold  together ; 
and  if  the  danger  persists,  the  surface  of  exposure  will  be 
still  further  reduced,  until  the  whole  plant  may  have  its 
leaves  completely  folded  up.     In  this  way  the  amount  of 


XEEOPHYTE  ASSOCIATIONS.  201 

reduction  of  the  exposed  surface  may  be  accurately  regu- 
lated to  suit  the  need  (see  §38). 

150.  Reduced  leaves. — In  regions  that  are  rather  per- 
manently dry,  it  is  observed  that  the  plants  in  general  pro- 
duce smaller  leaves  than  in  other  regions  (see  Fig.  173). 
That  this  holds  a  direct  relation  to  the  dry  conditions  is 


Fig.  173.  Leaves  from  the  common  basswood  (Tilia),  showing  the  effect  of  environ- 
ment ;  those  at  the  right  being  from  a  tree  growing  in  a  river  bottom  (mesophyte 
conditions) ;  those  at  the  left  being  from  a  tree  growing  upon  a  dune,  where  it  is 
exposed  to  intense  light,  heat,  cold,  and  wind.  Not  only  are  the  former  larger, 
but  they  are  much  thinner.  The  leaves  from  the  dune  tree  are  strikingly  smaller, 
much  thicker,  and  more  compact. — After  Cowles. 

evident  from  the  fact  that  the  same  plant  often  produces 
smaller  leaves  in  xerophytic  conditions  than  in  moist  con- 
ditions. One  of  the  most  striking  features  of  an  arid 
region  is  the  absence  of  large,  showy  leaves  (see  Fig.  172). 
These  reduced  leaves  are  of  various  forms,  such  as  the 
needle  leaves  of  pines,  or  the  thread-like  leaves  of  certain 
sedges  and  grasses,  or  the  narrow  leaves  with  inrolled 
margins  such  as  is  common  in  many  heath  plants.     The 


202 


PLANT   RELATIONS. 


Fig.  174.     Two  species  of  Achillea  on  different  soils.    The  one  to  the  left  was  grown 
In    drier    conditions    and    shows    an    abundant   development   of   hairs.— After 

SCHIMPER. 

extreme  of  leaf  reduction  has  been  reached  by  the  cactus 
plants,  whose  leaves,,  so  far  as  foliage  is  concerned,  have 
disappeared  entirely,  and   the  leaf  work  is   40ne   by  the 


XEKOPIIYTE  ASSOCIATIONS. 


203 


surface  of  the  globular,  cylindrical,  or  flattened  stems  (see 
§36). 

151.  Hairy  coverings. — A  covering  of  hairs  is  an  effective 
sun  screen,  and  it  is  very  common  to  find  plants  of  xerophyte 
regions  character- 
istically hairy  (see 
§35).  The  hairs 
are  dead  struc- 
tures, and  within 
them  there  is  air. 
This  causes  them 
to  reflect  the  light, 
and  hence  to  ap- 
pear white  or 
nearly  so.  This 
reflection  of  light 
by  the  hairs  dimin- 
ishes the  amount 
which  reaches  the 
working  region  of 
the  plant  (see  Fig. 
174). 

152.Bodyhabit. 
— Besides  the  va- 
rious   devices    for 
diminishing    ex- 
posure or  leaf  sur- 
face,   and     hence 
loss   of    water, 
enumerated  above, 
the  whole  habit  of 
the  plant  may  em- 
phasize the  same  purpose.    In  dry  regions  it  is  to  be  observed 
that  dwarf  growths  prevail,  so  that  the  plant  as  a  whole 
does  not  present  such  an  exposure  to  the  dry  air  as  in 
regions  of  greater  moisture  (see  Fig.  175).     Also  the  pros- 


Fig.  175.  Two  plants  of  a  common  scouring  rush  {Equi- 
setum),  showing  the  effect  of  environment ;  the  long, 
unbranched  one  having  grown  in  normal  mesophyte 
conditions  ;  the  short,  bushy  branching,  more  slender 
form  having  grown  on  the  dunes  (xerophyte  condi- 
tions).— After  Cowles. 


204 


PLANT   RELATIONS. 


trate  or  creeping  habit  is  a  much  less  exposed  one  in  such 
regions  than  the  erect  habit.  In  the  same  manner,  the  very 
characteristic  rosette  habit,  with  its  cluster  of  overlapping 
leaves  close  against  the  ground,  tends  to  diminish  loss  of 
water  through  transpiration. 

One  of  the  most  common  results  of  xerophytic  conditions 
upon  body  habit  is  the  development  of  thorns  and  spiny 


Fig.  176.    Young  plants  of  Euphorbia  splendem,  showing  a  development  of  thorns 
characteristic  of  the  plants  of  dry  regions. 


processes.  As  a  consequence,  the  vegetation  of  dry  regions 
is  characteristically  spiny.  In  many  cases  these  spiny  pro- 
cesses can  be  made  to  develop  into  ordinary  stems  or  leaves 
in  the  presence  of  more  favorable  water  conditions.  It  is 
probable,  therefore,  that  such  structures  represent  reduc- 
tions in  the  growth  of  certain  regions,  caused  by  the  unfavor- 
able conditions.  Incidentally  these  thorns  and  spiny  pro- 
cesses are  probably  of  great  service  as  a  protection  to  plants 
in  regions  where  vegetation  is  peculiarly  exposed  to  the 


XEK0PI1YTE  ASSOCIATIONS. 


205 


ravages  of  animals  (see  §105).  Examine  Figs.  176,  177, 
178,  179,  180,  181. 

153.  Anatomical  adaptations. — It  is  in  connection  with 
the  xerophytes  that  some  of  the  most  striking  anatomical 
adaptations  have  been 
developed.  In  such 
conditions  the  epider- 
mis is  apt  to  be  cov- 
ered by  layers  of 
cuticle,  which  are  de- 
veloped by  the  walls 
of  the  epidermal  cells, 
and  being  constantly 
formed  beneath,  the 
cuticle  may  become 
very  thick.  This 
forms  a  very  efficient 
protective  covering, 
and  has  a  tendency  to 
diminish  the  loss  of 
water  (see  §35).  It  is 
also  to  be  observed 
that  among  xerophytes 
there  is  a  strong  de- 
velopment of  palisade 
tissue.  The  working 
cells  of  the  leaves  next 
to  the  exposed  surface 
are  elongated,  and  are 
directed     endwise     to 

the  surface.  In  this  way  only  the  ends  of  the  elongated 
cells  are  exposed,  and  as  such  cells  stand  very  closely  to- 
gether, there  is  no  drying  air  between  them.  In  some 
cases  there  may  be  more  than  one  of  these  palisade  rows 
(see  §32).  It  has  been  observed  that  the  chloroplasts  in 
these  palisade  cells  are  able  to  assume  various  positions  in 


b  a. 

Fig.  177.  Two  plants  of  common  gorge  or  furze 
( Vlex),  showing  the  effect  of  environment :  b 
is  a  plant  grown  in  moist  conditions ;  a  is  a 
plant  grown  in  dry  conditions,  the  leaves  and 
branches  having  been  almost  entirely  developed 
as  thorns. — After  Lotheixer. 


206 


PLANT  RELATIONS. 


the  cell,  so  that  when 
the  light  is  very  intense 
they  move  to  the  more 
shaded  depths  of  the 
cell,  and  when  it  be- 
comes less  intense  they 
move  to  the  more  ex- 
ternal regions  of  the 
cell  (see  Fig.  182). 
The  stomata,  or  air 
pores,  which  are  devel- 
oped in  the  epidermis, 
are  also  great  regulators 
of  transpiration,  as  has 
been  mentioned  already 
(see  §31). 

154.  Water  reservoirs. 
— In   xero- 
phytes    at-  / 

tent  ion 
must  be 
given  not 
only  to  the 
regulation  of  transpiration,  but  also  to  the 
storage  of  water,  as  it  is  received  at  rare  inter- 
vals. It  is  very  common  to  find  a  certain  re- 
gion of  the  plant  body  given  over  to  this  work, 
forming  what  is  known  as  water  tissue.  In 
many  leaves  this  water  tissue  may  be  distin- 
guished from  the  ordinary  working  cells  by 
being  a  group  of  colorless  cells  (see  Figs.  183, 
184,  185).  In  plants  of  the  drier  regions  leaves 
may  become  thick  and  fleshy  through  acting 
as  water  reservoirs,  as  in  the  case  of  the  agave, 
sedums,  etc.  Fleshy  or  ' '  succulent "  leaves 
are  regarded  as  adaptations  of    prime  impor- 


Fig.  178.  A  brancli  of  Cytisus,  showing  the 
reduced  leaves  and  thorny  branches.— After 
Kerner. 


Fig.  179.  A 
leaf  of  traga- 
canth,  show- 
ing the  re- 
duced leaf- 
lets and  the 
thorn  -like 
tip.— After 
Kerner. 


XEEOPHYTE   ASSOCIATIONS. 


2o7 


tance  in  xerophytic  conditions.  In 
the  cactns  plants  the  peculiar  stems 
have  become  great  reservoirs  of 
moisture.  The  globular  body  may 
be  taken  to  represent  the  most  com- 
plete answer  to  this  general  problem, 
as  it  is  the  form  of  body  by  which 
the  least  amount  of  surface  may  be 
exposed  and  the  greatest  amount  of 
water  storage  secured.  In  the  case 
of  fleshy  leaves  and  fleshy  bodies  it 
has  long  been  noticed  that  they  not 
only  contain  water,  but  also  have  a 
great  power  of  re- 


Fig.  180.  A  fragment  of  bar- 
berry, showing  the  thorns. 
— After  Kerner. 


Fig.  181.  Twig  of  com- 
mon locust,  showing 
the  thorns.— After 
Kerner. 

15 


taining  it.  Plant 
collectors  have  found  much  difficulty  in 
drying  these  fleshy  forms,  some  of  which 
seem  to  be  able  to  retain  their  moisture  in- 
definitely, even  in  the  driest  conditions. 
155.  Xerophytic  structure. — The  adap- 
tations given  above  are  generally  found 
in  plants  growing  in  drouth  conditions, 
and  they  all  imply  an  effort  to  diminish 
transpiration.  It  must  not  be  supposed, 
however,  that  only  plants  living  in 
drouth  conditions  show  these  adapta- 
tions. Such  adaptations  result  in  what 
is  known  as  the  xerophytic  structure, 
and  such  a  structure  may  appear  even 
in  plants  growing  in  hydrophyte  condi- 
tions. For  example,  the  bulrush  grows 
in  shallow  water,  and  is  a  prominent 
member  of  one  of  the  hydrophyte  asso- 
ciations (see  §137) ;  and  yet  it  has  a  re- 
markably xerophytic  structure.  This  is 
probably  due  to  the  fact  that  although  it 


208 


PLANT  KELATIONS. 


stands  in  the  water  its  stem  is  exposed 
to  a  heat  that  is  often  intense. 

The  ordinary  prairie  (see  §169)  i3 
included  among  mesophyte  associa- 
tions on  account  of  the  rich,  well- 
watered  soil;  and  yet  many  of  the 
plants  are  very  xerophytic  in  struc- 
ture, probably  on  account  of  the  pre- 
vailing dry  winds. 

The  ordinary  sphagnum-bog  (see 
§140),  or  "  peat-bog,"  is  included 
among  hydrophyte  associations.  It 
has  an  abundance  of  water,  and  is  not 
exposed  to  blazing  heat,  as  in  the  case 
of  the  bulrushes,  or  to  drying  wind,  as 
in  the  case  of  prairie  plants ;  and  yet 
its  plants  show  a  xerophytic  structure. 
The  cause  for  this  has  not  yet  been 
determined,  although  several  sugges- 
tions have  been  made. 

It  is  evident,  therefore,  that  xero- 
phytic structures  are  not  necessarily 
confined  to  xerophytic  situations.  It 
is  probably  true  that  all  associations 
that  show  xerophytic 
structures  belong  to- 
gether more  naturally 
than  do  the  associa- 
tions that  are  grouped 
according  to  the  water 
supply. 

Associations. 

Xo  attempt  will  be 
made  to  classify  these 
very  numerous  associa- 


Fig.  182.  Cells  from  the  leaf 
of  a  quillwort  (Isoetes). 
The  light  is  striking  the 
cells  from  the  direction  of 
one  looking  at  the  illus- 
tration. If  it  be  some- 
what diffuse  the  chloro- 
plasts  distribute  them- 
selves through  the  shal- 
low cell,  as  in  the  cell  to 
the  left.  If  the  light  be 
intense,  the  chloroplasts 
move  to  the  wall  and  as- 
sume positions  less  ex- 
posed, as  in  the  cell  to 
the  right. 


Fig.  183.  A  section  through  a  Begonia  leaf ,  show- 
ing the  epidermis  (ep)  above  and  below,  the 
water-storage  tissue  (ws)  above  and  below,  and 
the  central  chlorophyll  region  (as). 


XEKOPHYTE  ASSOCIATIONS. 


209 


Fig.  184.  A  section  through  a  fleshy  leaf  (Clinid),  show- 
ing the  chlorophyll  region  on  the  outside  (shaded  and 
marked  as),  and  the  large  interior  water-storage  region 
(ws). 


tions,  but  a  few 
prominent  illus- 
trations will  be 
given. 

156.  Rock  as- 
sociations.— Vari- 
ous plants  are 
able  to  live  upon 
exposed  rock  sur- 
faces, and  there- 
fore form  distinct 
associations  of  xe- 
rophytes.  In  gen- 
eral they  are  lichens,  mosses,  and  crevice  plants  (see  Fig. 
186).  The  crevice  plants  are  those 
which  send  their  roots  into  the  rock 
crevices  and  so  gain  a  foothold. 
The  crevice  plants  also  commonly 
show  a  rosette  habit,  the  rosette  of 
overlapping  leaves  being  against  the 
rock  face,  and  therefore  in  the  most 
favorable  position  for  checking  loss 
of  Avater. 

157.  Sand  associations. — In  gen- 
eral, sand  associations  may  be  roughly 
grouped  as  beach  associations,  dune 
associations,  and  sandy  field  associa- 
tions. These  three  hold  a  certain 
definite  relation  to  one  another. 
This  natural  relationship  appears  on 
the  borders  of  the  large  lakes,  and  on 
seacoasts.  The  beach  is  nearest  the 
water,  the  dunes  are  next,  and  be- 
hind them  stretch  the  sandy  fields. 
When  the  three  types  are  thus  asso- 
ciated, the  plants  of  the   different 


Fig.  185.  A  section  through 
a  leaf  of  an  epiphyte, 
showing  a  very  large  de- 
velopment of  water  tissue 
between  the  upper  epi- 
dermis and  the  chloro- 
phyll region,  which  is 
restricted  to  near  the 
under  surface  of  the  leaf. 
—After  Schimper. 


210 


PLANT  RELATIONS. 


areas  pass  gradually  into  one  another.  It  is  very  common 
to  find  the  dunes  omitted  in  the  series,  and  to  have  the 
beaches  pass  gradually  into  the  sandy  fields. 

The  beach  association  is  usually  quite  characteristic,  and 
in  general  it  is  a  poor  flora,  the  beach  being  characteristic- 
ally bare.  The  plants  that  grow  in  such  conditions  are  apt 
to  occur  in  tufts,  or  are  creeping  plants.    It  is  evident  that 


Fig.  186.    A  rock  covered  with  lichens. 


while  the  water  may  seem  to  be  abundant,  it  disappears 
quickly,  so  that  plants  must  adapt  themselves  to  a  dry 
condition  of  the  soil,  which  is  poor  and  with  little  or  no 
accumulation  of  humus.  At  the  same  time,  the  exposure 
to  intense  light  i.s  extreme.  This  combination  results  in  a 
poor  display  of  individuals  and  of  species.  Here  and  there 
along  beaches,  where  special  conditions  have  favored  the 
accumulation  of  humus,  dense  vegetation  may  spring  up, 
but  it  should  not  be  confused  with  the  ordinary  beach  type. 


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212 


PLANT  RELATIONS. 


The  dune  associations  are  subjected  to  very  peculiar 
conditions.  Dunes  are  billows  of  sand  that  have  been  de- 
veloped by  prevailing  winds,  and  in  many  cases  they  are 
continually  changing  their  form  and  are  frequently  moving 


Fig.  188.  A  sandy  field  type,  showing  the  development  of  vegetation  upon  an  old 
beach.  The  vegetation  is  low,  often  tufted  and  heath-like,  being  composed  chiefly 
of  grasses,  bearberry  (Arctostaphylos)  and  Hudsonia.  In  the  background  to 
the  right  is  a  conifer  forest,  and  between  it  and  the  old  beach  is  seen  a  dense  mass 
of  bearberry,  a  very  characteristic  heath  plant,  and  forming  here  what  is  called  a 
transition  zone  between  the  beach  and  the  forest.— After  Cowles. 


landward  (see  Fig.  187).  The  moving  dunes  should  be 
distinguished  from  the  fixed  ones,  where  the  billow  form  is 
retained,  but  the  dunes  have  ceased  their  motion.  In  the 
case  of  the  active  dunes  a  peculiar  type  of  vegetation  is  de- 
manded.    As  is  to  be  expected,  the  flora  is  very  scanty,  and 


214  PLANT   RELATIONS. 

has  two  remarkably  developed  characters.  The  plants  are 
what  are  known  as  "  sand-binders/'  that  is,  the  underground 
structures  become  extremely  developed,  reaching  to  great 
distances  horizontally  and  vertically,  so  that  one  is  always 
surprised  at  the  extent  of  the  underground  system.  This 
wide  searching  for  water  results  in  giving  the  plants  a  deep 
anchorage  in  the  shifting  soil,  and  at  the  same  time  helps 
to  prevent  the  shifting.  As  soon  as  enough  of  the  sand- 
binders  have  established  tbemselves,  a  shifting  dune  becomes 
a  fixed  one.  Another  characteristic  that  must  be  strongly 
developed  by  these  plants  is  the  ability  to  grow  up  through 
the  sand  after  they  have  been  engulfed.  The  plants  of  the 
shifting  dunes  are  often  buried  as  the  dune  shifts,  and 
unless  the  burial  has  been  too  deep,  they  are  able  to  continue 
their  development  until  leaves  may  be  exposed  to  the  air. 
In  this  way  plants  have  often  developed  a  length  of  stem 
which  is  far  beyond  anything  they  attain  when  growing  in 
ordinary  conditions. 

The  sandy  field  associations  are  represented  by  a  much 
more  abundant  flora  than  the  beach  or  the  dune  associa- 
tions, the  general  character  being  tufted  grasses  and  low 
shrubby  growths  (see  Fig.  188). 

158.  Shrubby  heaths. — The  shrubby  heaths  are  very 
characteristic  of  the  more  northern  regions,  and  are  closely 
related  to  the  sandy  field  associations.  The  heath  soil  is 
apt  to  be  a  mixture  of  coarse  sand,  or  gravel  and  rock,  with 
an  occasional  deposit  of  humus,  and  would  be  regarded 
in  general  as  a  sterile  soil.  The  flora  of  the  shrubby 
heaths  shows  well-marked  strata,  the  upper  one  being  the 
low  shrubby  plants  of  the  heath  family,  most  prominent 
among  which  are  huckleberries  and  bearberries  (see  Fig. 
172).  The  lower  stratum  is  made  up  of  mosses  and 
Lichens.  A  branching  lichen,  usually  spoken  of  as  the 
"reindeer  moss,"  often  occurs  in  immense  patches  on 
such  heaths.  While  these  shrubby  heaths  occur  most 
extensively  towards   the  north,  small   areas  showing  the 


216  PLANT   RELATIONS. 

same  general  character  are  common  in  almost  all  temper- 
ate regions. 

159.  Plains. — Under  this  head  are  included  great  areas 
in  the  interior  of  continents,  where  dry  air  and  wind 
prevail.  The  plains  of  the  United  States  extend  from 
about  the  one  hundredth  meridian  westward  to  the  foot- 
hills of  the  Rocky  Mountains.  Similar  great  areas  are 
represented  by  the  steppes  of  Siberia,  and  in  the  interior  of 
all  continents.  These  regions  have  been  regarded  as  semi- 
desert  areas,  but  they  are  found  for  the  most  part  to  be 
far  from  the  real  desert  conditions.  They  are  certainly 
areas  of  comparative  dryness,  on  account  of  the  dry  winds 
which  prevail. 

Taking  the  plains  of  the  United  States  as  a  type,  a  very 
characteristic  plant  physiognomy  is  presented  (see  Fig. 
189).  In  general,  there  is  a  meadow-like  expanse,  but  the 
vegetation  is  much  more  sparse  than  in  meadows,  and  is 
much  more  dense  than  in  deserts.  The  two  characteristic 
plant  forms  are  the  bunch  grasses,  that  is,  grasses  which 
grow  in  great  tufts  ;  and  low  grayish  shrubs,  predomi- 
nantly "  sage  brush."  Under  the  shelter  of  the  sage  brush 
or  other  bush  forms,  many  low  herbs  succeed  in  growing. 
In  such  areas  the  growing  season  is  very  short,  during 
which  time  the  vegetation  looks  vigorous  and  fresh ;  but 
during  the  rest  of  the  year  it  is  very  dull.  In  some  parts 
the  plain  is  dry  enough  to  permit  the  growth  of  the  prickly- 
pear  cactus  (Opuntia),  which  may  take  possession  of  ex- 
tensive areas  (see  Fig.  190). 

Usually  there  are  two  rest  periods  during  the  year, 
developed  by  the  summer  drouth  and  the  winter  cold.  As 
a  consequence,  the  plants  of  the  area  are  partly  spring 
plants,  which  are  apt  to  be  very  brilliant  in  flower  ;  and 
partly  the  later,  deep-rooted  forms.  Over  such  areas  the 
transportation  of  seeds  by  the  wind  is  very  prominent,  as 
the  force  of  the  wind  and  the  freedom  of  its  sweep  make 
possible  very  wide  distribution.     It  is  in  such  areas  that 


£    K 


Fig.  192.  Two  plants  of  the  giant  cactus.  Note  the  fluted,  clumsy  branching,  leaf- 
less bodies  growing  from  the  rocky,  sterile  soil  characteristic  of  cactus  deserts. 
Certain  dry-ground  grasses  and  low,  shrubby  plants  with  small  leaves  may  be  seer 
in  the  foreground. 


220 


PLANT  KELATIONS. 


the  tumbleweed  habit  is  prominently  developed.  Certain 
low  and  densely  branching  plants  are  lightly  rooted  in 
the  soil,  so  that  at  the  close  of  their  growing  period  they 
are  easily  broken  off  by  the  wind,  and  are  rolled  to  great 


Fie.  194.    Tree-like  yuccas  from  the  arid  regions  of  Africa,  showing  the  very  numel 
OU6  thick  and  pointed,  sword-like  leaves. 


222  PLANT   RELATIONS. 

distances.  Where  some  barrier,  such  as  a  fence,  lies  across 
the  I  ruck  of  the  wind,  those  tumbleweeds  may  accumulate 
in  great  masses.  This  tumbling  over  the  surface  results 
in  an  extensive  scattering  of  seeds  (see  Fig.  L20). 

The  prairies,  so  characteristic  of  the  United  States,  are 
regarded  by  some  as  belonging  to  the  plains.  They  cer- 
tainly are  closely  related  to  them  in  origin,  but  can  hardly 
be  regarded  as  being  included  in  xerophyte  conditions,  as 
the  conditions  of  water  supply  and  soil  are  characteristically 
mesophyte,  under  which  head  they  will  be  considered. 

160.  Cactus  deserts.— In  passing  southward  on  the 
plains  of  the  United  States,  it  is  to  be  noted  that  the  con- 
ditions become  more  and  more  xerophytic,  and  that  the 
bunch  grasses  and  sage  brush,  peculiar  to  the  true  plains, 
gradually  merge  into  the  cactus  desert,  which  represents 
a  region  whose  conditions  are  intermediate  between  true 
plains  and  true  deserts  (see  Fig.  191).  In  the  United  States 
this  characteristic  desert  region  begins  to  appear  in  West- 
ern Texas,  New  Mexico,  Arizona,  and  Southern  California, 
and  stretches  far  down  into  the  Mexican  possessions.  This 
vast  arid  region  has  developed  a  peculiar  flora,  which  con- 
tains most  highly  specialized  xerophytic  forms.  The  va- 
rious cactus  forms  may  be  taken  as  most  characteristic, 
and  associated  with  them  are  the  agaves  and  the  yuccas. 
Not  only  are  the  adaptations  for  checking  transpiration 
and  for  retaining  water  of  the  most  extreme  kind,  but 
there  is  also  developed  a  remarkable  armature.  It  is  evi- 
dent that  such  succulent  bodies  as  these  plants  present 
might  speedily  disappear  through  the  attacks  of  animals, 
were  it  not  for  the  armor  of  spines  and  bristles  and  rigid 
walls.     Study  Figs.  38,  39,  40,  192,  193,  194. 

161.  Subtropical  deserts. — In  such  areas  xerophyte  con- 
ditions reach  the  greatest  extreme  in  the  combination  of 
maximum  heat  and  minimum  water  supply.  It  is  evident 
that  such  a  combination  is  almost  too  difficult  for  plants 
to  endure.     That  the  very  scanty  vegetation  is  due  to  lack 


I 


AY: 


-*V' 


Mmn 


I      1 


•. 


16 


224  PLANT   RELATIONS. 

of  water,  and  not  to  lack  of  proper  materials  in  the  soil,  is 
shown  by  the  fact  that  where  water  does  occur  oases  are 
developed,  in  which  luxuriant  vegetation  is  found. 

The  desert  which  extends  from  Egypt  across  Arabia  may 
be  regarded  as  a  typical  one.  It  is  to  be  noted  that  the 
vegetation  is  so  scanty  that  the  soil  is  the  conspicuous 
feature,  and  really  gives  the  characteristic  physiognomy 
(see  Fig.  1!)6).  Accordingly  the  appearance  of  the  deserts 
will  depend  upon  whether  the  desert  soil  is  rocky,  or  of 
small  stones,  or  gravel  (as  in  the  Desert  of  Sahara),  or  of 
red  clay,  or  of  the  dune  type.  As  is  to  be  expected,  such 
vegetation  as  does  occur  is  of  the  tuft  and  bunch  type,  as 
developed  by  certain  grasses,  or  of  the  low  irregular  bush 
type  (see  Fig.  195). 

In  the  South  African  deserts  certain  remarkable  plants 
hiive  been  noted  which  have  attained  a  certain  amount  of 
protection  through  mimicry,  rather  than  by  means  of  armor, 
as  in  the  case  of  the  cactus  forms.  Some  of  these  plants 
resemble  the  ordinary  stones  lying  about  upon  the  desert. 
With  the  subtropical  deserts  should  not  be  confused  such 
areas  as  those  about  the  Dead  Sea,  or  in  the  Death's  Valley 
in  Southern  California,  as  the  barrenness  of  these  areas  is 
due  to  the  strongly  alkaline  soils,  and  therefore  they  be- 
long to  the  saline  areas. 

162.  Thickets. — The  xerophyte  thicket  is  the  most 
strongly  developed  of  all  thicket  growths.  Mention  has 
been  made  of  willow  and  alder  thickets  in  hydrophyte  con- 
ditions, but  these  are  not  to  be  compared  in  real  thicket 
characters  with  the  xerophyte  thickets.  These  thickets 
are  especially  developed  in  the  tropics  and  subtropics,  and 
may  be  described  as  growths  which  are  scraggy,  thorny, 
and  impenetrable.  Warming  speaks  of  these  thickets  as 
"the  unsuccessful  attempt  of  Nature  to  form  a  forest." 
Evidently  the  conditions  are  not  quite  favorable  for  for- 
est development,  and  an  extensive  thicket  is  the  result. 
Such  thickets  are  well  developed  in  Texas,  where  they  are 


Fir;.  198.  A  xerophyte  conifer  forest  in  the  mountains.  The  peculiar  conifer  habit 
of  body  is  recognized,  the  trees  finding  foothold  in  the  crevices  of  rocks  or  in 
areas  of  rock  debris. 


XEROPHYTE  ASSOCIATIONS.  227 

spoken  of  as  "  chaparral."  These  chaparrals  are  notably- 
composed  of  mesquit  bushes,  acacias  and  mimosas  of  vari- 
ous sorts,  and  other  plants.  Similar  thickets  in  Africa  and 
Australia  are  frequently  spoken  of  as  "bush  "  or  "  scrub." 
In  all  of  these  cases  the  thicket  has  the  same  general  type, 
and  probably  represents  one  of  the  most  forbidding  areas 
for  travel. 

163.  Forests. — The  xerophyte  forest  associations  maybe 
roughly  characterized  under  three  general  heads : 

(1)  Coniferous  forests. — These  forests  are  very  common 
in  xerophyte  conditions  to  the  north,  and  also  in  the  more 
sterile  regions  towards  the  south  (see  Figs.  198  to  201). 
They  are  generally  spoken  of  as  evergreen  forests,  although 
the  name  is  not  distinctive.  These  forests  are  of  several 
types,  such  as  true  pine  forests,  in  which  pines  are  the 
prevailing  trees  and  the  shade  is  not  dense  ;  the  fir  and 
hemlock  forests,  which  are  relatively  dark  ;  and  the  mixed 
forests,  in  which  there  is  a  mingling  of  various  conifers. 
In  such  forests  the  soil  is  often  very  bare,  and  such  under- 
growth as  does  occur  is  largely  composed  of  perennial 
plants.  Many  characteristic  shrubs  with  fleshy  fruits  oc- 
cur, such  as  huckleberries,  bearberries,  junipers,  etc.  It 
will  be  noted  that  in  these  forests  a  characteristic  adapta- 
tion to  xerophyte  conditions  is  the  development  of  needle 
leaves,  which  are  not  only  narrow,  thus  presenting  a  small 
exposure  of  surface,  but  also  have  heavy  walls,  which 
further  prevents  excessive  transpiration. 

(2)  Foliage  forests.— -These  are  more  characteristic  of 
tropical  and  subtropical  xerophyte  regions.  Illustrations 
may  be  obtained  from  the  eucalyptus,  a  characteristic 
Australian  forest  tree,  the  live  oaks,  oleanders,  etc.  It 
will  be  noticed  that  in  these  cases  the  leaves  are  not  so 
narrow  as  the  needles  of  conifers,  but  are  generally  lance- 
shaped,  and  stiff  and  leathery,  indicating  heavy  walls  to 
reduce  transpiration. 

(3)  Leafless  forests.— In  Java  and  other  oriental  regions 


Fig.  199.— A  xerophyte  conifer  forest  in  the  Cumberland    Mountains    of    Tennessee. 
The  table  mountain  pines  find  footholds  in  crevices  of  the  rocks. 


Fig.  200.     A  pine  forest,  showing  the  slender,  tall,  continuous  trunks  and  compara- 
tively little  undergrowth.— After  Schimpeu. 


XEEOPHYTE   ASSOCIATIONS.  231 

areas  of  dry  naked  soil  are  sometimes  occupied  by  forest 
growths  that  show  no  development  of  leaves,  the  tree- 
like forms  appearing  continually  bare.  The  oriental  leaf- 
less tree  form  is  mostly  a  Casuarina.  Bordering  the  Gulf 
of  California,  both  in  Mexico  proper  and  in  Lower  Cali- 
fornia, there  are  leafless  forests  composed  of  various  kinds 
of  giant  cactus  (see  Fig.  192),  t  known  as  the  "cardon 
forests."  These  leafless  forests  represent  the  most  extreme 
xerophyte  conditions  occupied  by  plant  forms  that  may 
be  regarded  as  trees. 

164.  Salt  steppes. — In  addition  to  the  xerophyte  associa- 
tions enumerated  above,  in  which  the  water,  though  scanty, 
is  fresh,  the  two  following  may  be  considered.  The  soil 
and  air  are  relatively  dry,  as  in  ordinary  xerophytic  condi- 
tions, but  the  water  is  more  or  less  saturated  with  common 
salt  or  alkaline  salts.  The  salt  steppes  are  interior  arid 
wastes,  which  probably  mark  the  position  of  old  sea  basins. 
In  the  United  States  one  of  the  most  extensive  of  the  salt 
steppes  is  in  the  Great  Salt  Lake  basin  (see  Fig.  202).  It 
is  here  that  members  of  the  chenopod  family  are  especially 
at  home,  such  as  greasewoods,  seablights,  samphires,  etc., 
for  more  than  any  other  plants  have  they  learned  to  endure 
such  extremely  unfavorable  conditions.  An  extensive  alka- 
line waste  in  the  United  States  is  that  known  as  the  Bad 
Lands,  which  stretches  over  certain  portions  of  Nebraska 
and  Dakota,  and  in  which  the  waters  are  strongly  alkaline. 
165.  Salt  and  alkaline  deserts. — In  these  areas  the  water 
supply  reaches  its  minimum,  and  therefore  the  water  be- 
comes saturated  with  the  characteristic  salts  of  the  soil. 
ISTo  worse  combination  for  plant  activity  can  be  imagined 
than  the  combination  of  minimum  water  and  maximum 
salts.  In  consequence,  such  areas  are  almost,  if  not  abso- 
lutely, devoid  of  vegetation.  As  illustrations,  the  exten- 
sive desert  of  the  Dead  Sea  region  and  the  Death's  Valley 
in  Southern  California  may  be  cited. 


232 


PLANT  KELAT10NS 


165a.  Alpine  deserts. — In  alpine  regions  a  distinct  desert 
type  of  vegetation  appears  upon  the  drier  areas,  especially 
above  timber-line.  It  is  in  direct  contrast  with  the  alpine 
meadow  (§168),  which  is  developed  in  situations  where  the 


snow  can  lie.  On  the  steeper  slopes  there  is  no  accumula- 
tion of  snow,  and  the  scanty  vegetation  has  a  distinctly 
xerophytic  character  (Fig.  203). 


CHAPTER  XIV. 

MESOPHYTE    ASSOCIATIONS. 

1G6.  General  characters. — Mesophytes  make  up  tne  com- 
mon vegetation  of  temperate  regions,  the  vegetation  most 
commonly  met  and  studied.  The  conditions  of  moisture 
are  medium,  precipitation  is  in  general  evenly  distributed, 
and  the  soil  is  rich  in  humus.  The  conditions  are  not  ex- 
treme, and  therefore  special  adaptations,  such  as  are  neces- 
sary for  xerophyte  or  hydrophyte  conditions,  do  not  appear. 
This  may  be  regarded  as  the  normal  plant  condition.  It 
is  certainly  the  arable  condition,  and  most  adapted  to  the 
plants  which  men  seek  to  cultivate.  When  for  purposes 
of  cultivation  xerophyte  areas  are  irrigated,  or  hydrophyte 
areas  are  drained,  it  is  simply  to  bring  them  into  mesophyte 
conditions. 

In  looking  over  a  mesophyte  area  and  contrasting  it 
with  a  xerophyte  area,  one  of  the  first  things  evident  is  that 
the  former  is  far  richer  in  leaf  forms.  It  is  in  the  meso- 
phyte conditions  that  foliage  leaves  show  their  remarkable 
diversity.  In  hydrophyte  and  xerophyte  areas  they  are  apt 
to  be  more  or  less  monotonous  in  form.  Another  contrast 
is  found  in  the  dense  growth  over  mesophyte  areas,  much 
more  so  than  in  xerophyte  regions,  and  even  more  dense 
than  in  hydrophyte  areas. 

Among  the  mesophyte  associations  must  be  included  not 
merely  the  natural  ones,  but  those  new  associations  which 
have  been  formed  under  the  influence  of  man,  and  which 
do  not  appear  among  xerophyte  and  hydrophyte  associations. 

233 


MESOPHYTE   ASSOCIATIONS.     .  235 

These  new  associations  have  been  formed  by  the  introduc- 
tion of  weeds  and  culture  plants. 

167.  The  two  groups  of  associations. — Two  very  promi- 
nent types  of  associations  are  included  here  under  the  meso- 
phytes,  although  they  are  probably  as  distinct  from  one 
another  as  are  the  mesophyte  and  xerophyte  associations. 
One  group  is  composed  of  low  vegetation,  notably  the  com- 
mon grasses  and  herbs  ;  the  other  is  a  higher  woody  vegeta- 
tion, composed  of  shrubs  and  trees.  The  most  character- 
istic types  under  each  one  of  these  divisions  are  noted  as 
follows : 

A.   Grass  and  herb  associations. 

It  should  not  be  inferred  from  this  title  that  most 
grasses  are  not  herbs,  but  it  is  convenient  to  consider 
grasses  and  ordinary  herb  forms  separately. 

108.  Arctic  and  alpine  carpets. — These  are  dense  mats  of 
low  vegetation  occurring  beyond  forest  growth  in  arctic 
regions,  and  above  the  tree  limit  in  high  mountains.  These 
carpet-like  growths  are  a  notable  feature  of  such  regions. 
In  such  positions  the  growing  season  is  very  short,  and  the 
temperature  is  quite  low  at  times,  especially  at  night.  It 
is  evident,  therefore,  that  there  must  be  provision  for  rapid 
growth,  and  also  for  preventing  dangerous  radiation  of 
heat,  which  might  chill  the  active  plant  below  the  point  of 
safety.  It  is  further  evident  that  the  short  season  and  the 
low  temperature  form  a  combination  which  prevents  the 
growth  of  trees  or  shrubs,  or  even  tall  herbs,  because  the 
season  is  too  short  for  them  to  reach  a  protected  condition, 
and  their  more  exposed  young  structures  are  not  in  a  posi- 
tion to  withstand  the  daily  fall  of  temperature. 

These  carpets  of  vegetation  are  notably  fresh-looking, 
indicating  rapid  growth  ;  green,  indicating  an  abundance 
of  chlorophyll  and  great  activity ;  thick,  as  they  are 
mostly  perennials,  developed  from  abundant  underground 
structures  ;  low,  on  account  of  the  short  season  and  low 


Fig.  204.  Two  plants  of  a  rock-rose  (Helianthemum),  showing  the  effect  of  low 
ground  and  alpine  conditions.  The  low-ground  plant  (a)  shows  an  open  habit, 
and  elongated  stems  with  comparatively  large  and  well-separated  leaves.  The 
same  plant  in  alpine  conditions  is  drawn  to  the  same  scale  in  6,  and  magnified  in 
c,  the  very  short  and  compact  habit  heing  in  striking  contrast  with  that  of  the  low- 
ground  form.— After  Bonnier. 


MESOPHYTE  ASSOCIATIONS.  237 

temperature ;  and  soft,  the  low  stature  and  short  life  not 
involving  the  development  of  specially  rigid  structures  for 
support  or  resistance.  In  such  conditions,  as  would  be 
expected,  annuals  are  in  the  minority,  the  plants  being 
mostly  perennial  and  geophilous.  Geophilous  plants  are 
those  which  have  the  habit  of  disappearing  underground 
when  protection  is  needed.  This  is  probably  the  best  adap- 
tation for  total  disappearance  from  the  surface  and  for  rapid 
reappearance  (see  §146).  In  such  conditions,  also,  rosette 
forms  are  very  common,  the  overlapping  leaves  of  the  rosette 
closely  pressed  to  the  ground  diminishing  the  loss  of  heat 
by  radiation.  It  has  also  been  noticed  that  these  arctic  and 
alpine  carpets  show  intense  color  in  their  flowers,  and  often 
a  remarkable  size  of  flower  in  proportion  to  the  rest  of  the 
plant.  Wherever  the  area  is  relatively  moist,  the  carpet  is 
prevailingly  a  grass  mat ;  in  the  drier  and  sandier  spots  the 
herbs  predominate  (see  Figs.  202,  203). 

In  the  case  of  plants  which  can  grow  both  in  the  low 
ground  and  in  the  alpine  region,  a  remarkable  adaptation 
of  the  plant  body  to  the  different  conditions  may  be  noted. 
The  difference  in  appearance  is  sometimes  so  great  that  it 
is  hard  to  realize  that  the  two  plants  belong  to  the  same 
species  (see  Fig.  204). 

169.  Meadows. — This  term  must  be  restricted  to  natural 
meadow  areas,  and  should  not  be  confused  with  those  arti- 
ficial areas  under  the  control  of  man,  which  are  commonly 
mowed.  The  appearance  of  such  an  area  hardly  needs  defi- 
nition, as  it  is  a  well-known  mixture  of  grasses  and  flower- 
ing herbs,  the  former  usually  being  the  predominant  type. 
Such  meadow-like  expanses  are  common  in  connection  with 
forest  areas  (see  Fig.  205),  but  they  are  most  character- 
istically developed  on  flood-plains  along  streams.  In  most 
cases  the  local  meadow  is  probably  an  ephemeral  society,  to 
be  replaced  by  forest  growth. 

The  greatest  meadows  of  the  United  States  are  the  well- 
known  prairies,  which  extend  from  the  Missouri  eastward 


MESOPHYTE  ASSOCIATIONS.  239 

to  the  forest  regions  of  Illinois  and  Indiana  (see  Fig.  206). 
The  prairie  is  regarded  by  some  as  a  xerophytic  area,  and 
this  is  a  natural  conclusion  when  one  examines  only  the 
structures  of  the  plants  which  occupy  it.  It  is  certainly  a 
transition  area  between  the  plains  of  the  West  and  the  true 
mesophytic  areas  of  the  East,  and  there  is  a  general  tran- 
sition from  the  more  xerophytic  western  prairies  to  the 
more  mesophytic  eastern  prairies.  Moreover,  in  the  east- 
ern part  of  the  prairie  region  there  is  locally  every  grada- 
tion between  the  strongly  mesophytic  type  of  the  low  ground 
to  the  more  xerophytic  type  of  the  high  ground. 

The  vegetation  of  the  prairies  in  general  is  composed 
of  tufted  grasses  and  perennial  flowering  herbs.  Unfortu- 
nately, most  of  the  natural  prairie  has  disappeared,  to  be 
replaced  by  farms,  and  the  characteristic  prairie  forms  are 
not  easily  seen.  The  flowering  herbs  are  often  very  tall  and 
coarse,  but  with  brilliant  flowers,  such  as  species  of  aster, 
goldenrod,  rosin-weed,  indigo  plant,  lupine,  bush  clover,  etc. 
The  most  characteristic  of  these  forms  show  their  xero- 
phytic adaptations  by  their  rigidity  and  roughness. 

The  origin  of  the  prairie  has  long  been  a  vexed  question, 
which  has  usually  taken  the  form  of  an  inquiry  into  the 
conditions  which  forbid  the  growth  of  forests.  Prairies  are 
at  least  of  two  kinds.  Some  are  edaphic — that  is,  they  are 
due  to  local  soil  conditions.  Such  prairies  are  character- 
istic of  the  eastern  prairie  region,  and  even  appear  in  scat- 
tered patches  throughout  the  forest  region  as  far  east  as 
Ohio,  Kentucky,  etc.  They  are  probably  best  explained  as 
representing  old  swamp  areas,  which  at  a  still  more  ancient 
time  were  ponds  or  lakes.  All  the  prairies  of  the  Chicago 
area  are  evidently  edaphic,  being  associated  with  former 
extensions  of  Lake  Michigan.  Other  prairies  are  climatic — 
that  is,  they  are  due  to  general  climatic  conditions.  Such 
prairies  are  characteristic  of  the  western  prairie  region, 
merging  into  the  plains,  and  are  more  puzzling  than  the 
edaphic  prairies.  Among  the  several  explanations  sug- 
17 


k-L 


MESOPHYTE  ASSOCIATIONS.  241 

gested  perhaps  that  which  refers  the  western  prairies  to 
the  prevailing  dry  winds  is  the  most  prominent. 

The  extensive  plains  of  the  West  develop  the  strong 
and  dry  winds  which  prevail  over  this  prairie  region,  and 
this  brings  about  extremes  of  heat  and  drouth,  in  spite  of 
the  character  of  the  soil.  In  such  conditions  a  tree  in  a 
germinating  condition  could  not  establish  itself.  If  it  is 
protected  through  this  tender  period  it  can  maintain  itself 
afterward,  but  the  drying  winds  forbid  any  plant  with  a 
prolonged  and  sensitive  juvenile  period.  These  prairies, 
therefore,  would  represent  a  sort  of  broad  beach  between 
the  western  plains  and  the  eastern  prairies  and  forests. 

What  seems  to  be  a  confirmation  of  this  view  may  be 
observed  in  certain  north  and  south  valleys  in  the  Missouri 
region  which  lies  on  the  border  between  plains  and  prairies. 
The  eastern  slopes  of  such  valleys,  exposed  to  the  wind 
from  the  plains,  are  without  trees ;  while  on  the  western 
slopes,  protected  from  this  wind,  trees  occur. 

Probably  the  oldest  explanation  of  such  prairies  is  the 
occurrence  of  prairie  fires,  but  this  would  appear  to  be  toe 
local  a  cause  for  what  seems  to  be  a  continental  feature. 
Eecently,  however,  the  fire  theory  has  been  revived,  and 
evidence  has  been  brought  forward  to  show  that  in  some 
places,  at  least,  a  forest  growth  would  appear  if  fire  and 
stock  were  kept  out.  In  fact,  the  claim  is  made  that  Ne- 
braska is  becoming  gradually  forest-clad. 

170.  Pastures. — This  term  is  applied  to  areas  drier  than 
natural  meadows,  and  includes  the  meadows  formed  or  con- 
trolled by  man  (see  Fig.  207).  They  may  be  natural,  or 
derived  from  natural  meadow  areas,  or  from  forest  clear- 
ings ;  therefore  they  are  often  maintained  in  conditions 
which,  if  not  interfered  with,  would  not  produce  a  meadow. 
In  general,  the  pasture  differs  from  the  natural  meadow  in 
being  drier,  a  fact  often  due  to  drainage,  and  in  develop- 
ing lower  and  more  open  vegetation.  Naturally  the  plant 
forms  are  prevailingly  grasses,  and  their  cultivation  is  the 


242 


PLANT   RELATIONS. 


purpose  of  the  artificial  pasture,  but  the  meadow  tendency 
is  shown  by  the  coming  in  of  perennial  weeds.  The  inva- 
sion of  pastures  by  weeds  suggests  many  interesting  ques- 
tions.     Are  the  weeds  natives  or  foreigners?     Are   they 


Fig.  207.  A  juniper  heath  interspersed  with  pastures.  The  growths  of  juniper  are 
very  dense,  excluding  all  other  vegetation,  and  the  grass  or  pasture  areas  are  too 
dry  to  form  real  meadows. — After  Cowles. 


annuals  or  perennials  ?  What  is  the  relative  success  of  the 
different  invaders,  and  why  are  some  more  successful  than 
others  ?  A  study  of  pastures  will  also  reveal  the  fact  that 
there  is  great  difference  in  the  vegetation  of  mowed  and 
grazed  pastures.  The  same  effects  are  noted  when  natural 
meadows  are  used  for  grazing. 

B.    Woody  associations. 

These  associations  include  the  various  shrub  and  tree 
assemblages  of  mesophyte  areas,  assemblages  entirely  dis- 
tinct from  the  grass  and  herb  associations, 


MESOPHYTE   ASSOCIATIONS.  243 

171.  Thickets. — The  mesophyte  thickets  are  not  so 
abundant  or  impenetrable  as  the  xeropbyte  thickets.  They 
seem  to  be  developed  usually  as  forerunners  of  forest  vege- 
tation. An  illustration  of  this  fact  may  be  obtained  by 
noting  the  succession  of  plauts  which  appear  on  a  cleared 
area.  After  such  an  area  has  been  cleared  of  its  trees,  by 
cutting  or  by  fire,  it  is  overrun  by  herbs  that  develop 
rapidly  from  the  seed.  Sometimes  these  herbs  are  tall 
and  with  showy  flowers,  as  the  so-called  fire-weed  or  great 
willow  herb.  Following  the  herb  associations  there  is  a 
gradual  invasion  of  coarser  herbs  and  shrubby  plants, 
forming  thickets,  and  finally  a  forest  growth  may  appear 
again. 

In  arctic  and  alpine  mesophyte  regions  the  willow  is 
the  great  thicket  plant,  often  covering  large  areas,  but  in 
temperate  regions  willow  thickets  are  confined  to  stream 
banks  and  boggy  places,  being  the  characteristic  hydro- 
phyte thicket  form. 

The  upland  and  flood-plain  mesophyte  thickets  of  tem- 
perate regions  are  different  in  character.  For  example, 
the  upland  thicket  of  the  Northern  States  very  commonly 
contains  hazel,  birch,  and  aspen  as  dominant  plants ;  while 
the  flood-plain  thicket  is  apt  to  contain,  in  addition  to 
these,  prominent  growths  of  haws  and  wild  crab-apples. 
In  this  same  region  pure  thickets  frequently  occur — that 
is,  thickets  in  which  a  single  form  is  the  prevailing  type, 
as  pure  hazel  thickets  on  uplands,  or  pure  haw  thickets  on 
flood-plains. 

In  the  Southern  States  the  plants  enumerated  above 
may  not  be  the  characteristic  mesophyte  thicket  plants. 
For  example,  in  Kentucky  and  Tennessee  the  dominant 
thicket  plants  are  persimmon,  locust,  redbud,  and 
sassafras. 

172.  Forests  of  temperate  regions. — Deciduous  forests 
are  especially  characteristic  of  temperate  regions.  The 
deciduous  habit,  that  is,  the  habit  of  shedding  leaves  at  a 


244 


PLANT   RELATIONS. 


certain  period,  is  an  adaptation  to  climate.  In  the  tem- 
perate regions  the  adaptation  is  in  response  to  the  winter 
cold,  when  a  vast  reduction  of  delicate  exposed  surface  is 
necessary.  Instead  of  protecting  delicate  leaf  structures 
from  the  severe  cold  of  winter,  these  plants  have  formed 
the  habit  of  dropping  them  and  putting  out  new  leaves 
when  the  favorable  season  returns. 

It  is  instructive  to  notice  how  differently  the  conifers 
(pines,  etc.)  and  the  deciduous  trees  (oaks,  maples,  etc.)  have 

answered  the  problem  of  adaptation 
to  the  cold  of  winter.  The  conifers 
have  protected  their  leaves,  giving 
them  a  small  surface  and  heavy 
walls.  In  this  way  protection  has 
been  secured  at  the  expense  of 
working  power  during  the  season 
of  work.  Eeduced  surface  and 
thick  walls  are  both  obstacles  to 
leaf  work.  On  the  other  hand, 
the  deciduous  trees  have  devel- 
oped the  working  power  of  their 
leaves  to  the  greatest  extent,  giving 
them  large  surface  exposure  and 
comparatively  delicate  walls.  It 
is  out  of  the  question  to  protect 
such  an  amount  of  surface  during 
the  winter,  and  hence  the  decidu- 
ous habit.  The  conifers  are  saved 
the  annual  renewal  of  leaves,  but 
lose  in  working  power;  the  de- 
ciduous trees  must  renew  their  leaves  annually,  but  gain 
greatly  in  working  power. 

It  should  be  remarked  that  leaves  do  not  fall  because 
they  are  broken  off,  but  that  in  a  certain  sense  it  is  a 
process  of  growing  off.  Often  at  the  base  of  the  leaves, 
where  the   separation  is  to  occur,  a  cleavage  region  is 


Fig.  208 


A  section  through  the 
base  of  a  leaf  of  horse-chestnut 
preparing  to  fall  off  at  the  end 
of  the  growing  season.  A 
cleavage  plate  (s)  has  devel- 
oped between  the  woody  bun- 
dle (b)  and  the  surface.  Pres- 
ently this  reaches  the  surface, 
and  only  the  woody  strand 
fastens  the  leaf  to  the  stem. 


MESOPHYTE  ASSOCIATIONS.  245 

gradually  developed  until  the  leaf  is  entirely  separated 
from  the  stem  except  by  a  woody  strand  or  two,  which  is 
easily  broken  (see  Fig.  208).  In  this  way  the  scar  which 
remains  has  really  been  formed  before  the  leaf  falls. 

In  this  process  of  sloughing  off  leaves,  the  plant  cannot 
afford  to  lose  the  living  substance  present  in  the  working 
leaves.  This  substance,  during  the  preparation  for  the 
fall,  has  been  gradually  withdrawn  into  the  permanent 
parts  of  the  plant. 

It  will  be  noticed  that  in  general  deciduous  leaves  are 
thin,  exceedingly  variable  in  form,  and  in  a  general  hori- 
zontal position,  nor  do  they  have  the  firm,  leathery  texture 
of  the  xerophyte  leaves.  All  this  indicates  great  leaf  ac- 
tivity, for,  the  necessity  of  protection  being  removed,  the 
leaf  is  not  impeded  in  its  work  by  the  development  of  pro- 
tective structures. 

One  of  the  most  prominent  features  associated  with  the 
deciduous  habit  is  the  autumnal  coloration.  The  vivid 
colors  which  appear  in  the  leaves  of  many  trees,  just  before 
the  time  of  falling,  is  a  phenomenon  which  has  attracted  a 
great  deal  of  attention,  but  although  it  is  so  prominent,  the 
causes  for  it  are  very  obscure.  It  will  be  noticed  that  this 
autumnal  coloration  consists  in  the  development  of  various 
shades  of  two  typical  colors,  yellow  and  red.  These  colors 
are  often  associated  together  in  the  same  leaf,  and  some- 
times a  leaf  may  show  a  pure  color. 

The  two  colors  hold  a  very  different  relation  in  the  leaf 
cell.  It  is  known  that  the  yellow  is  due  to  the  breaking 
down  of  chlorophyll,  so  that  the  chloroplasts,  which  are 
green  when  active,  become  yellow  when  disorganizing,  and 
finally  bleach  out  entirely.  That  yellow  may  indicate  a 
post  mortem  change  of  chlorophyll  may  be  noticed  in  con- 
nection with  the  blanching  of  celery,  in  which  the  leaves 
and  upper  part  of  the  stem  may  be  green,  the  green  may 
shade  gradually  into  yellow,  and  finally  into  the  pure  white 
of  complete  blanching. 


246  PLANT  EELATIONS. 

The  red  shades,  however,  do  not  seem  to  hold  any  such 
relation  to  the  disorganization  of  chlorophyll.  The  red 
coloring  matter  appears  as  a  stain  in  the  cell  sap,  so  that 
what  might  be  called  the  atmosphere  of  the  active  cell  is 
suffused  with  red.  Certain  experiments  upon  plant  colors 
have  indicated  that  the  presence  of  the  red  color  slightly 
increases  the  temperature  by  absorbing  more  heat.  This 
has  suggested  that  the  red  color  may  be  a  slight  protec- 
tion to  the  living  substance,  which  has  ceased  working 
and  which  is  in  danger  of  exposure  to  cold.  If  this  be 
true,  it  may  be  that  the  same  explanation  will  cover  the 
case  of  the  red  flush  so  conspicuous  in  buds  and  young 
leaves  in  the  early  spring.  It  must  not  be  supposed  that 
the  need  of  protection  has  developed  the  color,  but  that 
since  it  is  developed  it  may  be  of  some  such  service  to  the 
plant.  The  whole  subject,  however,  is  too  indefinite  and 
obscure  to  be  presented  in  any  other  form  than  as  a  bare 
suggestion. 

Even  the  conditions  which  determine  autumnal  colora- 
tion have  not  been  made  out  certainly.  To  many  the  au- 
tumnal coloration  is  associated  with  the  coming  of  frost, 
which  simply  means  a  reduction  of  temperature ;  others 
associate  it  with  diminishing  water  supply ;  still  others 
associate  it  with  the  change  in  the  direction  of  the  rays  of 
light,  which  are  more  oblique  in  autumn  than  during  the 
active  growing  season.  It  is  certainly  true  that  the  colors 
are  far  more  brilliant  in  certain  years  than  in  others,  and 
that  the  coloration  must  be  connected  in  some  way  with 
the  food  relations  of  the  plants.  Eecent  experiments  have 
shown  that  the  red  coloration  is  largely  dependent  upon  low 
temperature,  which  affects  certain  of  the  food-stuffs,  and 
the  red  stain  is  one  of  the  products. 

The  autumnal  colors  are  notably  striking  in  American 
forests  on  account  of  the  fact  that  in  these  forests  there  is 
the  greatest  display  of  species,  and  hence  not  only  are  more 
colors  produced,  but  they  are  usually  strikingly  associated. 


MESOPHYTE  ASSOCIATIONS.  247 

Not  only  is  protection  during  the  cold  period  secured 
by  deciduous  forests  through  the  falling  of  leaves,  but  the 
development  of  scaly  buds  is  an  adaptation  to  the  same 
end.  By  means  of  these  overlapping,  often  hairy,  and  even 
varnished  structures,  delicate  growing  tips  are  protected 
during  the  cold  season.  The  development  of  cork,  also,  on 
the  older  parts,  is  a  measure  of  protection. 

Although  the  trees  are  the  dominant  plants  of  a  forest  as- 
sociation, it  must  not  be  forgotten  that  numerous  other  forms 
are  associated  with  them.  At  a  lower  level  stand  the  shrubs, 
below  these  the  tall  herbs,  then  the  low  herbs  and  grasses, 
and  finally  close  to  the  soil  mosses  and  lichens  occur. 
These  different  strata,  as  they  are  called,  represent  differ- 
ent habits  in  reference  to  light,  the  lower  strata  being  made 
up  of  shade  plants  as  compared  with  the  upper  strata.  In 
fact,  the  shade  habit  has  become  so  established  in  many 
plants  of  the  lower  strata  that  they  depend  upon  the  pres- 
ence of  the  overshadowing  strata,  and  could  not  live  with- 
out them. 

The  vernal  habit  is  also  an  interesting  feature  of  decidu- 
ous forests.  It  is  a  matter  of  common  observation  that  the 
rich  display  of  "  spring  flowers  "  occurs  in  forests  and  wooded 
glens  before  the  trees  come  into  full  foliage.  The  working 
season  of  these  vernal  plants  is  before  the  dense  foliage  of 
the  forest  shuts  off  the  light.  Accordingly,  they  are  mostly 
geophilous  in  habit  (see  §116),  sending  up  their  shoots  or 
leaves  with  great  rapidity  from  underground  tubers,  root- 
stocks,  etc.,  and  completing  their  vegetative  work  in  the 
short  period  during  which  the  light  is  available.  After  the 
forest  leaves  are  fully  developed  the  spring  flowers  disap- 
pear, waiting  in  their  subterranean  retreats  for  the  next 
short  period  of  activity.  Two  prominent  forms  of  the  ver- 
nal habit  may  be  observed.  The  leaves  may  appear  before 
the  flowers,  as  in  Erythronium  and  Hydrophyllum  ;  or  they 
may  appear  after  the  flowers,  as  in  Hepatica  and  Sanguinaria. 
One  of  the  wild  leeks  {Allium  tricoccum)  has  developed  a 


248  PLANT  EELATIONS. 

very  interesting  modification.  It  sends  up  its  rosette  of 
large  and  very  active  leaves  during  the  vernal  season,  and 
when  these  have  disappeared  the  flowers  are  developed  in 
the  forest  shade.  The  significance  of  this  is  that  while 
the  leaves  must  have  the  light  for  their  work,  the  flowers 
can  develop  just  as  well  in  the  shade. 

As  in  the  case  of  thickets,  deciduous  forests  may  be 
pure  or  mixed.  A  very  common  type  of  pure  forest  is  the 
beech  forest,  which  is  a  characteristic  dark  forest.  The 
wide-spreading  branches  of  neighboring  beeches  overlap 
each  other,  so  as  to  form  dense  shade.  As  a  consequence, 
in  a  pure  beech  forest  there  is  little  or  no  undergrowth ;  in 
fact,  no  lower  strata  of  vegetation  until  the  lowest  ones  are 
reached,  made  up  of  grasses  and  mosses.  Another  type  of 
pure  forest,  which  belongs  to  the  drier  regions,  is  the  oak 
forest,  which  forms  a  sharp  contrast  to  the  beech,  in  that 
it  is  a  light  forest,  permitting  access  of  light  for  lower 
strata  of  plants.  Hence  in  such  a  forest  there  is  usually  more 
or  less  undergrowth,  consisting  of  shrubs,  etc.,  which  may 
develop  regular  thickets.  The  typical  American  deciduous 
forest,  however,  is  the  great  mixed  forest,  made  up  of  many 
varieties  of  trees,  such  as  beech,  oak,  elm,  walnut,  hickory, 
gum,  maple,  etc. 

The  deciduous  forests  may  be  roughly  grouped  as  up- 
land and  flood-plain  forests,  the  former  being  less  luxuriant 
and  containing  fewer  types,  the  latter  being  the  highest  ex- 
pression of  forest  development  in  its  region.  A  few  general 
illustrations  may  be  given  as  follows  : 

In  northern  Illinois  the  upland  forest  is  mostly  made 
up  of  three  forms,  white  and  red  oaks  and  shellbark  hick- 
ory ;  while  the  flood-plain  forest  contains  twenty  to  twenty- 
five  tree  forms,  prominent  among  which  are  the  elms  (white 
and  slippery),  linden  (bass wood),  cottonwood,  ash,  silver 
maple,  box  elder,  walnut,  and  willows  (see  Fig.  211). 

Farther  south,  from  central  Illinois,  Indiana,  and  Ohio 
southward,  as  well  as  in  the  Alleghanies,  the  flood-plain  for- 


250  PLANT   RELATIONS. 

ests  are  the  richest  known,  containing,  in  addition  to  the 
forms  enumerated  above,  such  prominent  trees  as  the  syca- 
more, beech,  hackberry,  honey  locust,  coffee  tree,  sugar 
maple,  tulip  tree,  buckeye,  etc. 

In  Michigan  and  Wisconsin  the  upland  forests  consist 
prominently  of  beech,  sugar  maple,  and  hemlock,  a  charac- 
teristic mixture  of  deciduous  and  evergreen  trees ;  while 
the  flood-plain  forests  are  scarcely  at  all  developed. 

In  the  Alleghany  region  and  New  England  the  upland 
forests  are  very  extensive  and  complicated,  grading  from 
the  rich  flood-plain  forests  of  the  lower  levels  on  the  one 
hand,  to  the  strictly  xerophytic  forests  (pines  and  black 
oaks)  of  the  higher  levels  on  the  other  hand,  and  dominated 
by  various  oaks  (especially  white,  red,  and  chestnut  oaks), 
chestnuts,  and  hickories  (see  Figs.  209,  210). 

The  flood-plain  forests  of  New  England  are  not  so  rich 
as  those  of  the  Alleghany  region  and  Central  West,  the 
dominant  forms  being  elms,  linden,  ash,  maples,  sycamore, 
tulip  tree,  etc. 

173.  Tropical  forests. — The  tropical  forests  may  be 
grouped  under  two  general  heads :  (1)  the  evergreen  forests, 
and  (2)  the  deciduous  or  monsoon  forests.  The  former  are 
characterized  by  continuous  moisture,  and  are  most  largely 
developed  in  the  East  Indies  and  along  the  Amazon  and  its 
tributaries  in  South  America.  The  deciduous  tropical  for- 
ests are  characterized  by  having  a  period  of  relative  dry- 
ness, during  which  the  leaves  are  shed,  and  usually  border 
the  evergreen  forests. 

A.  Evergreen  forests. — These  rainy  forests  of  the  tropics 
may  be  regarded,  as  Warming  says,  "  as  the  climax  of  the 
world's  vegetation,"  for  the  conditions  in  which  they  are  de- 
veloped favor  constant  plant  activity  at  the  highest  possible 
pressure.  Such  great  forest  growths  are  found  within  the 
region  of  the  trade  winds,  where  there  is  heavy  rainfall, 
great  heat,  and  rich  black  soil.  So  abundant  is  the  precipi- 
tation that  the  air  is  often  saturated  and  the  plants  drip 


252  PLANT   RELATIONS. 

with  moisture.  In  such  conditions  pure  forests  may  oc- 
cur, characterized  by  such  tree  forms  as  the  tree  ferns, 
palms,  or  bamboos.  Only  the  great  mixed  tropical  forest 
will  be  considered.  The  main  characteristics  are  as  fol- 
lows : 

(1)  Absence  of  simultaneous  periodicity. — Perhaps  the 
most  striking  feature,  in  contrast  with  the  deciduous  for- 
ests, is  that  there  is  no  regular  period  for  the  develop- 
ment or  fall  of  leaves.  Leaf  activity  is  possible  through- 
out the  year,  and  there  is  no  time  of  bare  forest,  or  of 
forests  just  putting  out  leaves.  This  does  not  mean  that 
the  leaves  persist  indefinitely,  but  that  there  is  no  regular 
time  for  their  fall  and  formation.  Leaves  are  continually 
being  shed  and  formed,  but  the  trees  always  appear  in  full 
foliage. 

(2)  Density  of  growth. — Such  an  area  is  remarkably 
filled  with  vegetation  stratum,  after  stratum  occurring, 
resulting  in  gigantic  jungles.  The  higher  strata  may  be 
made  up  of  trees  of  different  heights,  below  them  are  shrubs 
of  varying  heights,  then  tall  and  low  herbs,  and  finally 
mosses  and  liverworts.  Among  these  close-set  standing 
forms,  great  vines  or  lianas  climb  and  bind  the  standing 
vegetation  into  an  inextricable  tangle  (see  Figs.  55,  212). 
In  addition  to  these,  hosts  of  aerial  plants  find  lodging 
places  upon  the  tree-trunks  and  vines  (see  Fig.  213).  These 
rainy  forests  of  the  tropics  furnish  the  very  best  conditions 
for  the  development  of  the  numerous  epiphytic  orchids, 
bromelias,  etc.  In  such  conditions  also  numerous  sapro- 
phytes occur.  Such  an  assemblage  of  vegetation  is  to  be 
found  nowhere  else. 

(3)  Number  of  species. — Not  only  is  there  an  immense 
number  of  individuals,  but  an  extraordinary  number  of 
species  occur.  A  list  of  plants  growing  in  these  forests 
would  show  a  remarkable  representation  of  the  plant 
kingdom. 

(4)  Forms  of  trees. — The  dense  vegetation  results  in 


MESOPHYTE   ASSOCIATIONS. 


255 


straight  leafless  tree-trunks,  so  that  the  leaves  of  trees  are 
mainly  clustered  at  the  tops  of  high  branches.  The  shade 
is  so  dense  and  the  interference  is  so  great  that  the  devel- 
opment of  low  branches  is  impossible.     It  is  common,  also, 


Fig.  213.  A  group  of  aerial  plants  (epiphytes)  from  a  tropical  forest.  Note  the  vari- 
ous habits  of  the  epiphytes  attached  to  the  tree-trunks,  and  the  dangling  roots.— 
After  Schimper. 


for  the  larger  trees  to  develop  a  system  of  buttresses  near 
the  base,  and  also  frequently  to  send  out  prop  roots  (see 
Figs.  100,  101). 

(5)  Absence  of  bud  scales. — In  the  deciduous  forest  bud 
scales  are  necessary  to  protect  the  tender  growing  tips  dur- 
ing  the  period  of  cold.     The  same  device  would  be  suffi- 
18 


256 


PLANT  RELATIONS. 


cient  to  protect  against  a  period  of  drouth.  In  the  tropical 
forest  there  is  danger  neither  from  cold  nor  drouth,  and  in 
such  conditions  bud  scales  are  not  developed,  and  the  buds 
remain  naked  and  unprotected. 

(6)  Devices  against  too  abundant  rain. — The  abundance 
of  rain  is  in  danger  of  checking  transpiration,  and  as  this 

process  is  essential  to  plant 
activity,  there  are  often 
found  devices  to  prevent 
the  leaves  from  becoming 
saturated.  Many  leaves 
have  cuticles  so  smooth 
and  glazed  that  the  water 
glances  off  without  soaking 
in  ;  in  other  cases  a  velvety 
covering  of  hairs  answers 
the  same  purpose;  in  still 
other  cases  leaves  are  gut- 
ter-pointed, that  is,  the  tip 
is  prolonged  as  a  sort  of 
gutter,  and  the  veins  are 
depressed,  the  whole  sur- 
face of  the  leaf  resembling 
a  drainage  system,  so  that 
the  rain  is  conducted  rap- 
idly from  the  surface  (see 
Fig.  214).  These  are  only 
a  few  illustrations  of  many 
devices  against  dangerous 
wetting. 

B.  Deciduous  or  mon- 
soon forests. — In  these  for- 
ests the  same  general  habits  prevail  as  in  the  rainy  evergreen 
forests,  but  to  a  less  degree.  For  example,  the  epiphytes 
and  lianas  are  present,  but  they  are  not  so  numerous  or 
conspicuous.     The    striking    difference,  however,  is    the 


Fig.  214.     A  gutter-pointed  leaf  from   a 
tropical  plant.— After  Schimper. 


MESOPHYTE  ASSOCIATIONS.  257 

deciduous  habit,  developed  apparently  by  the  regular 
recurrence  of  a  relatively  dry  period,  although  it  may  be 
very  short.  Such  forests  are  usually  adjacent  to  the  ever- 
green forests,  much  as  upland  forests  are  adjacent  to  flood- 
plain  forests. 


INDEX   TO   PLANT  RELATIONS. 


[The  italicized  numbers  indicate  that  the  subject  is  illustrated  on  the  page  cited. 
In  such  case  the  subject  may  be  referred  to  only  in  the  illustration,  or  it  may  be 
referred  to  also  in  the  test.] 


Acacia,  199. 

Achillea,  202. 

Adaptation,  147. 

Adiantum,  27. 

Aeration,  92,  93,  95,  183. 

Agave,  45,  47. 

Agrimony,  121. 

Ailanthus,  116. 

Air,  95,  98,  114,  122,  138. 

Air  cavities,  171,  172,  173,  175. 

Air  passages,  92,  93,  94,  95. 

Air  plants,   97,  98,  99,   100,   101, 

246. 
Air  roots,  97,  98,  99,  100. 
Alchemilla,  79. 
Algae,   1,   2,  87,  99,  107,  109,  110, 

111,  113,  150,  171,  172,  177. 
Alkaline  deserts,  255. 
Alpine  plants,  148,  231,  232,  233. 
Amicia,  9. 
Arapelopsis,  63. 
Aneraophilous,  122. 
Animals,  119,  121,  122,    123,  145, 

205. 
Annual  habit,  195. 
Annual  rings,  84. 
Anthurium,  97. 
Apple,  79. 
Araucaria,  74. 


Arbor  vitas,  139. 
Arctic  plants,  148,  232. 
Arrow-leaf,  186. 
Artillery  plant,  120. 
Ash,  116. 
Aspidium,  55. 
Assimilation,  154,  156. 
Autumn  coloration,  241. 

B 

Bacteria,  189. 
Banana,  88. 
Banyan,  105,  106. 
Barberry,  207. 
Bark,  84. 

Basswood,  116,  201. 
Beach,  209,  210. 
Beach  marshes,  253. 
Beach  meadows,  253 
Beach  pea,  118. 
Bean,  140. 
Bearberry,  212,  214. 
Beech  drops,  157. 
Beech  forest,  145,  243. 
Beggar  ticks,  119,  121. 
Begonia,  25,  208. 
Bellflower,  19,  80. 
Bidens,  119. 
Bignonia,  115. 


260 


INDEX. 


Bilbergia,  136. 

Birches,  71. 

Black  moss,  96,  101. 

Bladderwort,  178. 

Blade,  35. 

Bloodroot,  195. 

Bogs,  143. 

Box  elder,  84. 

Bramble,  94. 

Branched  leaves,  19,  20,  21,  23. 

Buds,  70,  73,  75,  141,  247. 

Bulbs,  73,  75,  81. 

Bulrush,  142,  148,  185, 186,  207. 

Burdock,  121, 122. 

Bush,  227. 

Bush  clover,  43. 

Buttercup,  185. 

Buttresses,  103,  104. 


C 


Cactus  deserts,  217,  222. 

Cactus  forms,  45,  46,  47,  146,  202, 

207,  215,  216,  217,  218,  219,  222. 
Calyx,  78,  79,  80, 125. 
Campanula,  19,  80. 
Caoutchouc,  136. 
Carbohydrates,  153,  156. 
Carbon,  153. 

Carbon  dioxide,  30,  151,  153. 
Cardon  forests,  231. 
Carnation,  4%- 
Carnivorous  plants,  155,  156,  157, 

173,  189. 
Carpel,  78,  79,  80, 125. 
Carrot,  120. 
Castor-oil  bean,  73. 
Casuarina,  231. 
Catalpa,  117. 
Catchfly,  136. 

Cat-tail  flag,  142,  148,  185,  186. 
Cercis,  10. 


Change  in  temperature,  145. 
Chaparral,  227. 
Chlorophyll,  6,  8,  149,  152. 
Chloroplasts,  39, 107,  152,  205, 208, 

209,  245. 
Chrysanthemum,  23. 
Cilia,  109,  111. 
Claytonia,  196. 
Cleistogamous,  130. 
Clematis,  113. 
Climbing  stems,  60,  61,  62,  63,  64, 

102. 
Clinging  roots,  99,  102. 
Clinia,  209. 
Cocklebur,  120, 121. 
Compass  plants,  10,  12,  197,  198. 
Compound  leaves,  19,  20,  21,  23. 
Conducting  tissue,  171. 
Conifer  forests,  226,  227,  228,  229, 

230. 
Conifers,  83,  190,  191,  225,  226. 
Cork,  247. 
Corn,  85,  90. 
Corolla,  78,  79,  80. 
Cortex,  83,  84,  93,  94,  107,  108. 
Cottonwood,  70. 
Cotyledons,  50,  51,  73,  139, 140. 
Crevice  plants,  94,  209. 
Cuticle,  42,  205. 
Cycad,  22. 
Cycloloma,  117. 
Cypress  knees,  95,  96,  183. 
Cypripedium,  132,   133,  134,  185, 

136. 
Cytisus,  206. 


D 

Dandelion,  82,  114,  117. 
Darlingtonia,  157. 
Date  palm,  86. 
Dead-nettle,  80. 


INDEX. 


261 


Deciduous  forests,  243. 

Deciduous  habit,  143,  196,  243, 244. 

Deserts,  221,  222,  223. 

Desiccation,  194. 

Desmodium  gyrans,  49. 

Destruction  of  plants,  148. 

Diatoms,  174. 

Dicotyledons,  35,  83,  116. 

Differentiation,  3. 

Digestion,  154,  156. 

Dionasa,  160, 161. 

Dodder,  106,  107,  157. 

Dog-tooth  violet,  144. 

Dragon  tree,  15. 

Drainage,  143,  145. 

Drosera,  158,  159. 

Drouth,  193. 

Duckweed,  97,  175. 

Dunes,  145,  201,  209,  211,  212. 

Dwarf  growths,  203. 


E 


Easter  lily,  14. 

Echeveria,  17. 

Ecological  factors,  163. 

Ecology,  4,  149. 

Eel  grass,  I84. 

Egg,  110,  111. 

Elaters,  118. 

Elatine,  93. 

Elm,  63,  67,  68,  75. 

Embryo,  111,  139. 

Entomophilous,  122,  123. 

Epidermis,  37,  40,  41,  4?,  83,  84, 

107,  170,  205,  208,  209. 
Epilobium,  112,  113,  128,  135. 
Epiphyte,  209. 
Equisetum,  111,  203. 
Erect  stems,  62,  65,  66,  67,  68,  69, 

70,  71. 
Erica,  200. 


Erythronium,  144. 
Euphorbia,  204. 


F 

Ferns,  55,  56,  85,  88,  100,  111,  113, 

119. 
Ficus,  8. 

Figwort,  128,  135. 
Fireweed,  112,  113,  128,  135,  243. 
Fittonia,  37,  152. 
Fixed  light  position,  197. 
Flag,  126,  133. 
Floating  stems,  59. 
Floats,  171,  172,  173. 
Flowers,  76,  78,  140. 
Foliage  forests,  227. 
Foliage  leaves,  6,  28,  139. 
Forest  clearing,  143,  145. 
Forests,  190,  226,  227,  228,  229. 
Fruit,  113,  114,  115,  116,  117,  118, 

119,  120,  121,  122. 
Pucus,  171. 
Functions,  3. 
Fungi,  87,  107,  109,  110. 
Furze,  205. 


G 


Galium,  121. 

Gamete,  110,  111,  112,  113. 

Geophilous  habit,  55,  56,  73,  74,  75, 

76,  77,  78,  81,  195,  196,  237. 
Geotropism,  69,  91,  138. 
Germination,  111,  138,  139, 140. 
Gorse,  205. 
Grape  vine,  61. 
Grass,  187,  197,  216,  236. 
Gravity,  91. 
Guard  cells,  38. 
Gymnosperms,  115. 


262 


INDEX. 


H 

Habenaria,  127. 

Hairs,  43,  92,  136,  146, 202,  203. 

Harebell,  19,  80. 

Hawthorn,  36. 

Heart-wood,  151. 

Heat,  112,  138,  145,  164. 

Heath  plants,  189,  200,  214. 

Helianthemum,  236. 

Heliotropism,    12,  13,   68,   72,  78, 

139. 
Hemlock,  190. 
Horse-chestnut,  244- 
Hosts,  106. 
House  leek,  19. 
Houstonia,  129,  135. 
Huckleberry,  214. 
Hudsonia,  212. 
Hura  crepitans,  120. 
Hydrophytes,  168,  170,  174. 
Hydrotropism,  91,  138. 


Insects  and  flowers,  123. 
Iris,  126,  133. 
Isoetes,  94,  95,  208. 
Ivy,  99. 


Juncus,  77. 
Juniper,  51, 


V#. 


Lactuca,  12,  197,  198. 
Lady-slipper,    132,   133, 
136. 


134,  135, 


Lakes,  143,  148. 

Laminaria,  177. 

Larch,  178,  190. 

Latex,  136. 

Leafless  forests,  227. 

Leaflet,  19. 

Leaf-relation,  53. 

Lemna,  97. 

Lespedeza,  43. 

Lianas,  60,  61,  62,  63,  64,  102,  245, 

246. 
Lichens,  194,  209,  214. 
Life-relations,  4,  7,  8,  53,  77. 
Light,  143,  167,  197. 
Light-relation,  7,  8. 
Lily,  38,  40. 
Live-for-ever,  18. 
Live  oak,  101. 
Liverworts,  118. 
Locomotion,  113. 
Locust,  207. 
Long  moss,  96, 101. 
Loosestrife,  130,  135. 
Lotus,  180. 

M 

Mangroves,  192,  192a. 
Maple,  26,  115,  116. 
Maranta,  38. 
Marchantia,  107. 
Meadows,  237,  238. 
Mechanical  tissue,  172. 
Mesophyll,  38,  39,  40  41,  42, 152. 
Mesophytes,  168,  233. 
Migration,  58,  75,  147. 
Mildew,  109,  157. 
Milkweed,  117. 
Mimosa,  199. 
Mistletoe,  107. 
Mold,  109. 

Monocotyledons,  35,   85,  88,   116, 
186. 


INDEX. 


263 


Moors,  187,  188. 

Mosaic  arrangement,  25,  27,  37. 

Mosses,  87,  107,  110,  113,  118,  188, 

194,  209,  214. 
Motile  leaves,  9,   10,  11,  49,  198, 

199. 
Mould,  109. 
Mullein,  43,  44. 
Mushrooms,  157. 


N 


Nectar,  123,  158. 
Nelumbium,  180. 
Nicotiana,  80. 
Nightshade,  26. 
Nitrogen,  153. 
Nodes,  54. 
Nuphar,  92. 
Nutrition,  3,  149. 
Nymphaea,  178,  180. 


0 


Oak,  69,  101. 

Oak  forest,  145,  248. 

GMogonium,  111. 

Orchids,  98,  99,  126,  127,  132, 133, 

134,  135,  136,  189. 
Organs,  3. 
Ornithogalum,  81. 
Ovary,  79,  80,  125. 
Ovules,  78,  79,  SO. 
Oxalis,  10,  50,  199. 
Oxygen,  29,  138,  153. 


Palisade  tissue,  39,  40,  42,  205. 
Palms,  86,  87,  230. 
Pandanus,  103. 


Parasites,  106,  150. 

Passion  vine,  62. 

Pastures,  241,  242. 

Pellionia,  24. 

Pentstemon,  137. 

Peony,  78. 

Petals,  78,  79,  80. 

Petioles,  15,  26,  35,  55. 

Phlox,  80. 

Photosynthesis,   28,   29,   150,   152, 

153,  156. 
Physiology,  149. 
Pickerel  weed,  181,  182. 
Pines,  63,  65,  66,  112,  115,  117,  190, 

227,  229, 230. 
Pirus,  79. 
Pistil,  77,  79,  80. 
Pitcher  plant,  155,  156,  157,  158. 
Pith,  83,  84,  107. 
Plains,  213,  215,  216. 
Plankton,  174. 
Plant  body,  2. 
Plastid,  152. 
Platycerium,  100. 
Plumes,  112,  113,  114,  116,  117. 
Plumule,  51,  140. 
Pollen,    77,    111,    112,    115,    121, 

123. 
Pollination,  77,  115,  122,  123. 
Polygonatum,  35. 
Ponds,  142,  175,  178,  180,  I84. 
Pondweed,  176,  181,  182. 
Potato,  74,  76. 
Potentilla,  43,  79. 
Prairies,  208,  222,  237,  240. 
Prickles,  146. 

Prickly  lettuce,  12,  197,  198. 
Primrose,  137. 
Procumbent  stem,  57. 
Profile  position,  197,  198. 
Pronuba,  130,  131. 
Prop  roots,  99,  103,  104, 105,  106, 

255. 


264 


INDEX. 


Protandry,  128,  135. 

Protection  of  leaves,  9, 10, 11,  12, 

41,  42,  43,  48,  49. 
Proteids,  153,  156,  189. 
Protogyny,  128,  135. 
Protoplasm,  154,  156. 
Ptelea,  115. 
Puff-balls,  157. 


Quillwort,  94,  95,  208. 


R 


Rain,  51,  256. 

Ranunculus,  185. 

Raspberry,  91. 

Receptacle,  79,  81,  114. 

Redbud,  10. 

Reed  grass,  142,  185, 186. 

Reed  swamps,  185. 

Reproduction,  3,  109. 

Respiration,  32,  154,  156. 

Rhizoids,  107. 

Rivalry,  146. 

Robinia,  125,  126,  133,  207. 

Rock-rose,  236. 

Rock  associations,  209,  210. 

Roots,  89,  90,  95,  98,  99,  138,  139, 

171. 
Root-cap,  108. 
Root-hairs,  90. 
Rootstalk,  55,  56,  75,   76,  77,  78, 

195. 
Rose  acacia,  125,  126,  133. 
Rosette  habit,  16,  17,  18,  19,  47,  94, 

158,  160,  209,  237. 
Rosinweed,  10,  197,  198. 
Rubber  tree,  104. 
Runners,  57,  93. 
Rusts,  157. 


S 


Sage  brush,  216. 

Sagittaria,  186. 

Saintpaulia,  16. 

Salt  deserts,  231. 

Salt  steppes,  231. 

Sand  associations,  209. 

Sandy  fields,  209,  212. 

Sanguinaria,  195. 

Saprophytes,  150,  189. 

Sap-wood,  151. 

Sargassum,  172. 

Sarracenia,  155, 156,  158. 

Saxifrage,  58. 

Scale  leaves,  70,  75. 

Scales,  141. 

Scouring  rush,  203. 

Screw  pine,  103. 

Scrub,  227. 

Seaweeds,  1,  2,  87,  99. 

Sedges,  187. 

Seed-dispersal,  112,  113,  114,  116, 

117, 118,  119,  120. 
Seed-plants,  111,  119,  121. 
Seeds,  111,  112, 113,  115,  138,  139, 

140. 
Selaginella,  26,  100,  194. 
Sempervivum,  19. 
Senecio,  114. 

Sensitive  plants,  11,  48,  50,  199. 
Sepals,  78,  79,  80. 
Shepherdia,  44- 
Shoots,  53. 

Silphium,  10,  197,  198. 
Smilax,  61. 
Snapdragon,  80,  137. 
Soil.  90,  94,   145,   151,    166,    214, 

224. 
Solomon's  seal,  35,  76. 
Spanish  needle,  119,  121. 
Sphagnum,  188. 


INDEX. 


265 


Sphagnum-bogs,  208. 

Sphagnum- moors,  188. 

Spines,  146,  204. 

Spirogyra,  110. 

Spongy  tissue,  39, 1ft. 

Spore  case,  55,  118,  119. 

Spore-dispersal,  109,  111,  112,  113, 

114,  118. 
Spores,  109,  110,  111,  112. 
Spring  beauty,  196. 
Spring  plants,  143, 144. 
Squash  seedlings,  50. 
Squirting  cucumber,  120. 
Staghorn  fern,  100. 
Stamens,  78,  79,  80,  125. 
Starch,  153. 
Star  cucumber,  61. 
Star-of-Bethlehem,  81. 
Stem,  54,  83,  139. 
Steppes,  216. 
Stigma,  80,  125. 
Stipules,  35. 
Stomata,  38,  40,  206. 
Strawberry  plant,  57,  58,  93. 
Struggle  for  existence,  142. 
Style,  80,  125. 
Subterranean  stems,  54,  55,  56,  76, 

77,  78. 
Succulent  plants,  222. 
Sugar,  153. 
Sundew,  158,  159. 
Sunflower,  72. 
Swamp-forest,  190,  191. 
Swamps,  183,  187. 
Swamp-thickets,  188. 


Tamarack,  178,  190. 
Tap  root,  93. 
Taxus,  42. 


Teasel,  136. 

Telegraph  plant,  49. 

Temperature,  145. 

Tendrils,  61,  62,  63. 

Thallus,  107. 

Thickets,  188,  224,  243. 

Thistle,  117. 

Thorns,   146,   204,  205,  206,  207. 

224. 
Thuja,  139. 
Tilia,  116,  201. 
Tillandsia,  96, 101. 
Toad-flax,  80. 
Toadstools,  149. 
Tobacco,  80. 
Touch-me-not,  119. 
Tragacanth,  206. 

Transpiration,  31,  33,  154,  193,256. 
Tropical  forest,  254- 
Trumpet  creeper,  99. 
Tubers,  74,  76,  196. 
Tumbleweeds,  117,  220. 
Turf-building,  185. 


Ulex,  205. 
Ulothrix,  109,  111. 
Utricularia,  173,  174. 


Vallisneria,  184- 

Vascular  bundles,  83,  84,  92, 

94,  107,  108,  151,  171. 
Vegetative  multiplication,  109. 
Veins,  35,  36,  37,  40,  151. 
Velamen,  99. 
Venation,  35, 36,  37. 
Victoria,  180. 
Violet,  117,  119. 


266 


INDEX. 


W 

Walnut,  82. 

Water,  90,    92,  94,  95,   113,   138, 
142,  151, 163,  193,  206,  250. 

Water  lily,  178, 180,  181. 

Water  reservoirs,  206,  208,  209. 

Weeds,  147. 

Willow,  35,  243. 

Wind,  95,  98,  114, 122,  .167. 

Wings,  112,  115, 116. 


Witch  hazel,  118,  119. 
Woodbine,  61,  63. 


Xerophytes,  168,  193,  208. 
Xerophytic  structure,  207. 


Yew,  42. 

Yucca,  45,  47,  130, 131, 


TWENTIETH    CENTURY   TEXT-BOOKS 


PLANT  STRUCTURES 

A  SECOND   BOOK  OF  BOTANY 


BY 

JOHN   M.   COULTER,   A.M.,   Ph.D. 

HEAD   OF    DEPARTMENT   OF   BOTANY 
UNIVERSITY    OF    CHICAGO 


SECOND   EDITION  REVISED 


NEW    YORK 
D.    APPLETON    AND    COMPANY 

1910 


Copyright,  1899,  1904, 
Bt  D.  APPLETON  AND  COMPANY 


PKEFACE 


In  the  preface  to  Plant  Relations  the  author  gave  his 
reasons  for  suggesting  that  the  ecological  standpoint  is  best 
adapted  for  the  first  contact  with  plants.  It  may  be,  how- 
ever, that  many  teachers  will  prefer  to  begin  with  the  mor- 
phological standpoint,  as  given  in  the  present  book.  Eec- 
ognizing  this  fact,  Plant  Structures  has  been  made  an 
independent  volume  that  may  precede  or  follow  the  other, 
or  may  provide  a  brief  course  of  botanical  study  in  itself. 

Although  in  the  present  volume  Morphology  is  the  domi- 
nant subject,  it  seems  wise  to  give  a  somewhat  general  view 
of  plants,  and  therefore  Physiology,  Ecology,  and  Taxonomy 
are  included  in  a  general  way.  For  fear  that  Physiology 
and  Ecology  may  be  lost  sight  of  as  distinct  subjects,  and 
to  introduce  important  topics  not  included  in  the  body  of 
the  work,  short  chapters  are  devoted  to  them,  which  seek 
to  bring  together  the  main  facts,  and  to  call  attention  to 
the  larger  fields. 

This  book  is  not  a  laboratory  guide,  but  is  for  reading 
and  study  in  connection  with  laboratory  work.  An  accom- 
panying pamphlet  for  teachers  gives  helpful  suggestions 
to  those  who  are  not  already  familiar  with  its  scope  and 
purpose.  It  is  not  expected  that  all  the  forms  and  sub- 
jects presented  in  the  text  can  be  included  in  the  labora- 
tory exercises,  but  it  is  believed  that  the  book  will  prove  a 
useful  companion  in  connection  with  such  exercises.  It 
is  very  necessary  to  co-ordinate  the  results  of  laboratory 
work,  to  refer  to  a  larger  range  of  material  than  can  be 
handled,  and  to  develop  some  philosophical  conception  of 


^  PREFACE 

the  plant  kingdom.  The  learning  of  methods  and  the 
collection  of  facts  are  fundamental  processes,  but  they 
must  be  supplemented  by  information  and  ideas  to  be  of 
most  service. 

The  author  does  not  believe  in  the  use  of  technical 
terms  unless  absolutely  necessary,  for  they  lead  frequently 
to  mistaking  definitions  of  words  for  knowledge  of  things. 
But  it  is  necessary  to  introduce  the  student  not  merely  to 
the  main  facts  but  also  to  the  literature  of  botany.  Ac- 
cordingly, the  most  commonly  used  technical  terms  are 
introduced,  often  two  or  three  for  the  same  thing,  but  it 
is  hoped  that  familiarity  with  them  will  enable  the  student 
to  read  any  ordinary  botanical  text.  Care  has  been  taken 
to  give  definitions  and  derivations,  and  to  call  repeated 
attention  to  synonymous  terms,  so  that  there  may  be  no 
confusion.  The  chaotic  state  of  morphological  terminology 
tempted  the  author  to  formulate  or  accept  some  consistent 
scheme  of  terms ;  but  it  was  felt  that  this  would  impose 
upon  the  student  too  great  difficulty  in  reading  far  more 
important  current  texts. 

Chapters  I-XII  form  a  connected  whole,  presenting  the 
general  story  of  the  evolution  of  plants  from  the  lowest  to 
the  highest.  The  remaining  chapters  are  supplementary, 
and  can  be  used  as  time  or  inclination  permits,  but  it  is  the 
judgment  of  the  author  that  they  should  be  included  if 
possible.  The  flower  is  so  conspicuous  and  important  a 
feature  in  connection  with  the  highest  plants,  that  Chapter 
XIII  seems  to  be  a  fitting  sequel  to  the  preceding  chapters. 
It  also  seems  desirable  to  develop  some  knowledge  of  the 
great  Angiosperm  families,  as  presented  in  Chapter  XIV, 
since  they  are  the  most  conspicuous  members  of  every  flora. 
In  this  connection,  the  author  has  been  in  the  habit  of 
directing  the  examination  of  characteristic  flowers,  and  of 
teaching  the  use  of  ordinary  taxonomic  manuals.  Chap- 
ter XV  deals  with  anatomical  matters,  but  the  structures 
included  are  so  bound  up  with  the  form  and  work  of  plants 


PREFACE 


Vll 


that  it  seems  important  to  find  a  place  for  them  even  in  an 
elementary  work.  The  reason  for  Chapters  XVI  and  XVII 
has  been  stated  already,  and  even  if  Plant  Relations  is  stud- 
ied, Chapter  XVII  will  be  useful  either  as  a  review  or  as  an 
introduction.  In  the  chapter  on  Plant  Physiology  the 
author  has  been  guided  by  Noll's  excellent  resume  in  the 
"  Strasburger  "  Botany. 

The  illustrations  have  been  entirely  in  the  charge  of 
Dr.  Otis  W.  Caldwell,  who  for  several  years  has  conducted 
in  the  University,  and  in  a  most  efficient  way,  such  labo- 
ratory work  as  this  volume  implies.  Many  original  illus- 
trations have  been  prepared  by  him,  and  under  his  direction 
by  Messrs.  S.  M.  Coulter,  B.  A.  Goldberger,  W.  J.  G.  Land, 
and  A.  C.  Moore,  and  some  are  credited  to  Dr.  Chamberlain 
and  Dr.  Cowles,  of  the  University,  but  it  is  a  matter  of 
regret  that  pressure  of  work  and  time  limitation  have  for- 
bidden a  still  greater  number.  The  authors  of  the  original 
illustrations  are  cited,  and  where  illustrations  have  been 
obtained  elsewhere  the  sources  are  indicated. 

The  author  would  again  call  attention  to  the  fact  that 
this  book  is  merely  intended  to  serve  as  a  compact  supple- 
ment to  three  far  more  important  factors :  the  teacher,  the 
laboratory,  and  field  work.  John  M.  Coulter. 

The  University  of  Chicago,  November,  1899. 

PREFACE  TO   THE  REVISED   EDITION 

During  the  last  five  years  the  science  of  Botany  has 

made  rapid  progress,  both  in  the  addition  of  new  facts  and 

in  changed  points  of  view.     Some  of  this  progress  affects 

Plant  Structures,  and  it  is  recorded  in  this  revised  edition 

so  far  as  it  can  be  without  a  complete  rewriting  of  the 

volume.     Changes  will  be  found,  therefore,  in  statements 

of  fact,  in  points  of  view,  in  terminology,  in  illustrations, 

and  also  in  the  addition  of  new  material. 

John  M.  Coulter. 

The  University  of  Chicago,  April,  1904, 
19 


CONTENTS 

CHAPTER  PAGE 

I. — Introduction 1 

II. — Thallophytes  :  Alg^e 4 

III. — The  evolution  of  sex 12 

IV. — The  great  groups  of  Alg^e     ...  17 

V. — Thallophytes  :  Fungi 48 

VI. — The  food  of  plants 83 

VII. — Bryophytes 93 

VIII. — The  great  groups  of  Bryophytes 109 

IX. — Pteridophytes 128 

X. — The  great  groups  of  Pteridophytes      ....  155 

XL — Spermatophytes  :  Gymnosperms 171 

XII. — Spermatophytes  :  Angiosperms 195 

XIII.— The  flower 218 

XIV. — Monocotyledons  and  dicotyledons 232 

XV. — Differentiation  of  tissues 280 

XVI. — Plant  physiology 297 

XVII. — Plant  ecology 311 

Glossary 329 

Index 337 


BOTANY 

PART    II.— PLANT    STRUCTURES 


CHAPTEE    I 

INTRODUCTION 

1.  Differences  in  structure. — It  is  evident,  even  to  the 
casual  observer,  that  plants  differ  very  much  in  structure. 
They  differ  not  merely  in  form  and  size,  but  also  in  com- 
plexity. Some  plants  are  simple,  others  are  complex,  and 
the  former  are  regarded  as  of  lower  rank. 

Beginning  with  the  simplest  plants — that  is,  those  of 
lowest  rank — one  can  pass  by  almost  insensible  grada- 
tions to  those  of  highest  rank.  At  certain  points  in  this 
advance  notable  interruptions  of  the  continuity  are  dis- 
covered, structures,  and  hence  certain  habits  of  work,  chang- 
ing decidedly,  and  these  breaks  enable  one  to  organize  the 
vast  array  of  plants  into  groups.  Some  of  the  breaks  ap- 
pear to  be  more  important  than  others,  and  opinions  may 
differ  as  to  those  of  chief  importance,  but  it  is  customary 
to  select  three  of  them  as  indicating  the  division  of  the 
plant  kingdom  into  four  great  groups. 

2.  The  great  groups. — The  four  great  groups  may  be 
indicated  here,  but  it  must  be  remembered  that  their  names 
mean  nothing  until  plants  representing  them  have  been 
studied.     It  will  be  noticed  that  all  the  names  have  the 

1 


2  PLANT   STRUCTURES 

constant  termination  phytes,  which  is  a  Greek  word  mean- 
ing "  plants."  The  prefix  in  each  case  is  also  a  Greek  word 
intended  to  indicate  the  kind  of  plants. 

(1)  Thallophytes. — The  name  means  "thallus  plants," 
but  just  what  a  "thallus"  is  can  not  well  be  explained 
until  some  of  the  plants  have  been  examined.  In  this 
great  group  are  included  some  of  the  simplest  forms, 
known  as  Algce  and  Fungi,  the  former  represented  by  green 
thready  growths  in  fresh  water  and  the  great  host  of  sea- 
weeds, the  latter  by  moulds,  mushrooms,  etc. 

(2)  Bryophytes. — The  name  means  "  moss  plants,"  and 
suggests  very  definitely  the  forms  which  are  included. 
Every  one  knows  mosses  in  a  general  way,  but  associated 
with  them  in  this  great  group  are  the  allied  liverworts, 
which  are  very  common  but  not  so  generally  known. 

(3)  Pteridophytes. — The  name  means  "  fern  plants,"  and 
ferns  are  well  known.  Not  all  Pteridophytes,  however,  are 
ferns,  for  associated  with  them  are  the  horsetails  (scouring 
rushes)  and  the  club  mosses. 

(4)  Spermatophytes. — The  name  means  "  seed  plants  " — 
that  is,  those  plants  which  produce  seeds.  In  a  general 
way  these  are  the  most  familiar  plants,  and  are  commonly 
spoken  of  as  "  flowering  plants."  They  are  the  highest  in 
rank  and  the  most  conspicuous,  and  hence  have  received 
much  attention.  In  former  times  the  study  of  botany  in 
the  schools  was  restricted  to  the  examination  of  this  one 
group,  to  the  entire  neglect  of  the  other  three  great  groups. 

3.  Increasing  complexity. — At  the  very  outset  it  is  well 
to  remember  that  the  Thallophytes  contain  the  simplest 
plants — those  whose  bodies  have  developed  no  organs  for 
special  work,  and  that  as  one  advances  through  higher 
Thallophytes,  Bryophytes,  and  Pteridophytes,  there  is  a  con- 
stant increase  in  the  complexity  of  the  plant  body,  until  in 
the  Spermatophytes  it  becomes  most  highly  organized,  with 
numerous  structures  set  apart  for  special  work,  just  as  in  the 
highest  animals  limbs,  eyes,  ears,  bones,  muscles,  nerves,  etc.. 


INTRODUCTION  3 

are  set  apart  for  special  work.  The  increasing  complexity 
is  usually  spoken  of  as  differentiation — that  is,  the  setting 
apart  of  structures  for  different  kinds  of  work.  Hence  the 
Bryophytes  are  said  to  be  more  highly  differentiated  than 
the  Thallophytes,  and  the  Spermatophytes  are  regarded  as 
the  most  highly  differentiated  group  of  plants. 

4.  Nutrition  and  reproduction. — However  variable  plants 
may  be  in  complexity,  they  all  do  the  same  general  kind  of 
work.  Increasing  complexity  simply  means  an  attempt  to 
do  this  work  more  effectively.  It  is  plant  work  that  makes 
plant  structures  significant,  and  hence  in  this  book  no  at- 
tempt will  be  made  to  separate  them.  All  the  work  of 
plants  may  be  put  under  two  heads,  nutrition  and  repro- 
duction, the  former  including  all  those  processes  by  which 
a  plant  maintains  itself,  the  latter  those  processes  by  which 
it  produces  new  plants.  In  the  lowest  plants,  these  two 
great  kinds  of  work,  or  functions,  as  they  are  called,  are 
not  set  apart  in  different  regions  of  the  body,  but  usually 
the  first  step  toward  differentiation  is  to  set  apart  the  re- 
productive function  from  the  nutritive,  and  to  develop 
special  reproductive  organs  which  are  entirely  distinct  from 
the  general  nutritive  body. 

5.  The  evolution  of  plants. — It  is  generally  supposed  that 
the  more  complex  plants  have  descended  from  the  simpler 
ones ;  that  the  Bryophytes  have  been  derived  from  the  Thallo- 
phytes, and  so  on.  All  the  groups,  therefore,  are  supposed 
to  be  related  among  themselves  in  some  way,  and  it  is  one 
of  the  great  problems  of  botany  to  discover  these  relation- 
ships. This  theory  of  the  relationship  of  plant  groups  is 
known  as  the  theory  of  descent,  or  more  generally  as  evo- 
lution. To  understand  any  higher  group  one  must  study 
the  lower  ones  related  to  it,  and  therefore  the  attempt  of 
this  book  will  be  to  trace  the  evolution  of  the  plant  king- 
dom, by  beginning  with  the  simplest  forms  and  noting  the 
gradual  increase  in  complexity  until  the  highest  forms  are 
reached. 


CHAPTEK  II 

THALLOPHYTES  :  ALG-53 

6.  General  characters. — Thallophytes  are  the  simplest  of 
plants,  often  so  small  as  to  escape  general  observation,  but 
sometimes  with  large  bodies.  They  occur  everywhere  in 
large  numbers,  and  are  of  special  interest  as  representing 
the  beginnings  of  the  plant  kingdom.  In  this  group  also 
there  are  organized  all  of  the  principal  activities  of  plants, 
so  that  a  study  of  Thallophytes  furnishes  a  clew  to  the 
structures  and  functions  of  the  higher,  more  complex 
groups. 

The  word  "thallus"  refers  to  the  nutritive  body,  or 
vegetative  body,  as  it  is  often  called.  This  body  does  not 
differentiate  special  nutritive  organs,  such  as  the  leaves  and 
roots  of  higher  plants,  but  all  of  its  regions  are  alike.  Its 
natural  position  also  is  not  erect,  but  prone.  While  most 
Thallophytes  have  thallus  bodies,  in  some  of  them,  as  in 
certain  marine  forms,  the  nutritive  body  differentiates  into 
regions  which  resemble  leaves,  stems,  and  roots  ;  also  cer- 
tain Bryophytes  have  thallus  bodies.  The  thallus  body, 
therefore,  is  not  always  a  distinctive  mark  of  Thallophytes, 
but  must  be  supplemented  by  other  characters  to  determine 
the  group. 

7.  Algae  and  Fungi.— It  is  convenient  to  separate  Thallo- 
phytes into  two  great  divisions,  known  as  Algcs  and  Fungi. 
It  should  be  known  that  this  is  a  very  general  division  and 
not  a  technical  one,  for  there  are  groups  of  Thallophytes 
which  can  not  be  regarded  as  strictly  either  Algae  or  Fungi, 
but  for  the  present  these  groups  may  be  included. 


THALLOPHYTES:  ALG^E  5 

The  great  distinction  between  these  two  divisions  of 
Thallophytes  is  that  the  Algae  contain  chlorophyll  and  the 
Fungi  do  not.  Chlorophyll  is  the  characteristic  green  color- 
ing matter  found  in  plants,  the  word  meaning  "  leaf  green." 
It  may  be  thought  that  to  use  this  coloring  material  as  the 
basis  of  such  an  important  division  is  somewhat  superficial, 
but  it  should  be  known  that  the  presence  of  chlorophyll  gives 
a  peculiar  power — one  which  affects  the  whole  structure 
of  the  nutritive  body  and  the  habit  of  life.  The  presence 
of  chlorophyll  means  that  the  plant  can  make  its  own  food, 
can  live  independent  of  other  plants  and  animals.  Algae, 
therefore,  are  the  independent  Thallophytes,  so  far  as  their 
food  is  concerned,  for  they  can  manufacture  it  out  of  the 
inorganic  materials  about  them. 

The  Fungi,  on  the  other  hand,  contain  no  chlorophyll, 
can  not  manufacture  food  from  inorganic  material,  and 
hence  must  obtain  it  already  manufactured  by  plants  or 
animals.  In  this  sense  they  are  dependent  upon  other  or- 
ganisms, and  this  dependence  has  led  to  great  changes  in 
structure  and  habit  of  life. 

It  is  supposed  that  Fungi  have  descended  from  Algae — 
that  is,  that  they  were  once  Algae,  which  gradually  acquired 
the  habit  of  obtaining  food  already  manufactured,  lost  their 
chlorophyll,  and  became  absolutely  dependent  and  more  or 
less  modified  in  structure.  Fungi  may  be  regarded,  there- 
fore, as  reduced  relatives  of  the  Algae,  of  equal  rank  so  far 
as  birth  and  structure  go,  but  of  very  different  habits. 

ALG.E 

8.  General  characters. — As  already  defined,  Algae  are 
Thallophytes  which  contain  chlorophyll,  and  are  therefore 
able  to  manufacture  food  from  inorganic  material.  They 
are  known  in  general  as  "  seaweeds,"  although  there  are 
fresh-water  forms  as  well  as  marine.  They  are  exceedingly 
variable  in  size,  ranging  from  forms  visible  only  by  means 


6  PLANT   STRUCT DEES 

of  the  compound  microscope  to  marine  forms  with  enor- 
mously bulky  bodies.  In  general  they  are  hydrophytes — that 
is,  plants  adapted  to  life  in  water  or  in  very  moist  places. 
The  special  interest  connected  with  the  group  is  that  it  is 
supposed  to  be  the  ancestral  group  of  the  plant  kingdom — 
the  one  from  which  the  higher  groups  have  been  more  or 
less  directly  derived.  In  this  regard  they  differ  from  the 
Fungi,  which  are  not  supposed  to  be  responsible  for  any 
higher  groups. 

9.  The  subdivisions. — Although  all  the  Algae  contain 
chlorophyll,  some  of  them  do  not  appear  green.  In  some 
of  them  another  coloring  matter  is  associated  with  the  chlo- 
rophyll and  may  mask  it  entirely.  Advantage  is  taken  of 
these  color  associations  to  separate  Algae  into  subdivisions. 
As  these  colors  are  accompanied  by  constant  differences  in 
structure  and  work,  the  distinction  on  the  basis  of  colors  is 
more  real  than  it  might  appear.  Upon  this  basis  four  sub- 
divisions may  be  made.  The  constant  termination  phycew, 
which  appears  in  the  names,  is  a  Greek  word  meaning  "  sea- 
weed," which  is  the  common  name  for  Algae ;  while  the  pre- 
fix in  each  case  is  the  Greek  name  for  the  color  which  char- 
acterizes the  group. 

The  four  subdivisions  are  as  follows :  (1)  Cyanophycece, 
or  "  Blue  Algae,"  but  usually  called  "  Blue-green  Algae,"  as  the 
characteristic  blue  does  not  entirely  mask  the  green,  and 
the  general  tint  is  bluish-green  ;  (2)  Ohlorophycece,  or  "  Green 
Algae,"  in  which  there  is  no  special  coloring  matter  associ- 
ated with  the  chlorophyll ;  (3)  Phceophycece,  or  "  Brown 
Algae  " ;  and  (4)  Rhodophycece,  or  "  Ked  Algae." 

It  should  be  remarked  that  probably  the  Cyanophyceae 
do  not  belong  with  the  other  groups,  but  it  is  convenient  to 
present  them  in  this  connection. 

10.  The  plant  body. — By  this  phrase  is  meant  the  nutri- 
tive or  vegetative  body.  There  is  in  plants  a  unit  of  struc- 
ture known  as  the  cell.  The  bodies  of  the  simplest  plants 
consist  of  but  one  cell,  while  the  bodies  of  the  most  com- 


THALLOPHYTES:   ALG^E 


Mzz- 


B 


G-' 


plex  plants  consist  of  very  many  cells.  It  is  necessary  to 
know  something  of  the  ordinary  living  plant  cell  before  the 
bodies  of  Algae  or  any  other  plant  bodies  can  be  under- 
stood. 

Such  a  cell  if  free  is  approximately  spherical  in  outline, 
(Fig.  6),  but  if  pressed  upon  by  contiguous  cells  may  become 
variously  modified  in  form 
(Fig.  1).  Bounding  it  there 
is  a  thin,  elastic  wall,  com- 
posed of  a  substance  called 
cellulose.  The  cell  wall, 
therefore,  forms  a  delicate 
sac,  which  contains  the  liv- 
ing substance  known  as  pro- 
toplasm. This  is  the  sub- 
stance which  manifests  life, 
and  is  the  only  substance 
in  the  plant  which  is  alive. 
It  is  the  protoplasm  which 
has  organized  the  cellulose 
wall  about  itself,  and  which 
does  all  the  plant  work.  It 
is  a  fluid  substance  which 
varies  much  in  its  consistence,  sometimes  being  a  thin  vis- 
cous fluid,  like  the  white  of  an  egg,  sometimes  much  more 
dense  and  compactly  organized. 

The  protoplasm  of  the  cell  is  organized  into  various 
structures  which  are  called  organs  of  the  cell,  each  organ 
having  one  or  more  special  functions.  One  of  the  most 
conspicuous  organs  of  the  living  cell  is  the  single  nucleus,  a 
comparatively  compact  and  usually  spherical  protoplasmic 
body,  and  generally  centrally  placed  within  the  cell  (Fig.  1). 
All  about  the  nucleus,  and  filling  up  the  general  cavity 
within  the  cell  wall,  is  an  organized  mass  of  much  thinner 
protoplasm,  known  as  cytoplasm.  The  cytoplasm  seems  to 
form  the  general  background  or  matrix  of  the  cell,  and  the 


Fig.  1.  Cells  from  a  moss  leaf,  showing 
nucleus  (B)  in  which  there  is  a  nucle- 
olus, cytoplasm  (C),  and  chloroplasts 
(A). — Caldwell. 


3  PLANT  STRUCTURES 

nucleus  lies  imbedded  within  it  (Fig.  1).  Every  working 
cell  consists  of  at  least  cytoplasm  and  nucleus.  Sometimes 
the  cellulose  wall  is  absent,  and  the  cell  then  consists  sim- 
ply of  a  nucleus  with  more  or  less  cytoplasm  organized 
about  it,  and  is  said  to  be  naked. 

Another  protoplasmic  organ  of  the  cell,  very  conspicuous 
among  the  Algae  and  other  groups,  is  the  plastid.  Plastids 
are  relatively  compact  bodies,  commonly  spherical,  variable 
in  number,  and  lie  imbedded  in  the  cytoplasm.  There  are 
various  kinds  of  plastids,  the  most  common  being  the  one 
which  contains  the  chlorophyll  and  hence  is  stained  green. 
The  chlorophyll-containing  plastid  is  known  as  the  chloro- 
plastid,  or  chloroplast  (Fig.  1).  An  ordinary  alga-cell,  there- 
fore, consists  of  a  cell  wall,  within  which  the  protoplasm  is 
organized  into  cytoplasm,  nucleus,  and  chloroplasts. 

The  bodies  of  the  simplest  Algae  consist  of  one  such 
cell,  and  it  may  be  regarded  as  the  simplest  form  of  plant 
body.  Starting  with  such  forms,  one  direction  of  advance 
in  complexity  is  to  organize  several  such  cells  into  a  loose 
row,  which  resembles  a  chain  (Fig.  4) ;  in  other  forms  the 
cells  in  a  row  become  more  compacted  and  flattened,  form- 
ing a  simple  filament  (Figs.  2,  5) ;  in  still  other  forms  the 
original  filament  puts  out  branches  like  itself,  producing 
a  branching  filament  (Fig.  8).  These  filamentous  bodies 
are  very  characteristic  of  the  Algae. 

Starting  again  with  the  one-celled  body,  another  line  of 
advance  is  for  several  cells  to  organize  in  two  directions, 
forming  a  plate  of  cells.  Still  another  line  of  advance  is  for 
the  cells  to  organize  in  three  directions,  forming  a  mass  of 
cells. 

The  bodies  of  Algae,  therefore,  may  be  said  to  be  one- 
celled  in  the  simplest  forms,  and  in  the  most  complex  forms 
they  become  filaments,  plates,  or  masses  of  cells. 

11.  Reproduction. — In  addition  to  the  work  of  nutrition, 
the  plant  body  must  organize  for  reproduction.  Just  as  the 
nutritive  body  begins  in  the  lowest  forms  with  a  single  cell 


THALLOPHYTES:   ALG^E  o, 

and  becomes  more  complex  in  the  higher  forms,  so  repro- 
duction begins  in  very  simple  fashion  and  gradually  be- 
comes more  complex.  Two  general  types  of  reproduction 
are  employed  by  the  Algae,  and  all  other  plants.  They  are 
as  follows : 

(1)  Vegetative  multiplication. — This  is  the  only  type  of 
reproduction  employed  by  the  lowest  Algae,  but  it  persists 
in  all  higher  groups  even  when  the  other  method  has  been 
introduced.  In  this  type  no  special  reproductive  bodies  are 
formed,  but  the  ordinary  vegetative  body  is  used  for  the 
purpose.  For  example,  if  the  body  consists  of  one  cell,  that 
cell  cuts  itself  into  two,  each  half  grows  and  rounds  off  as 
a  distinct  cell,  and  two  new  bodies  appear  where  there  was 
one  before  (Figs.  3,  6).  This  process  of  cell  division  is  very 
complicated  and  important,  involving  a  division  of  nucleus 
and  cytoplasm  so  that  the  new  cells  may  be  organized  just 
as  was  the  old  one.  Wherever  ordinary  nutritive  cells  are 
used  directly  to  produce  new  plant  bodies  the  process  is 
vegetative  multiplication.  This  method  of  reproduction  may 
be  indicated  by  a  formula  as  follows  :  P  —  P  —  P  —  P  —  P,  in 
which  P  stands  for  the  plant,  the  formula  indicating  that 
a  succession  of  plants  may  arise  directly  from  one  another 
without  the  interposition  of  any  special  structure. 

(2)  Spores. — Spores  are  cells  which  are  specially  organ- 
ized to  reproduce,  and  are  not  at  all  concerned  in  the  nutri- 
tive work  of  the  plant.  Spores  are  all  alike  in  their  power 
of  reproduction,  but  they  are  formed  in  two  very  distinct 
ways.  It  must  be  remembered  that  these  two  types  of 
spores  are  alike  in  power  but  different  in  origin. 

Asexual  spores. — These  cells  are  formed  by  cell  divi- 
sion. A  cell  of  the  plant  body  is  selected  for  the  purpose, 
and  usually  its  contents  divide  and  form  a  variable  number 
of  new  cells  within  the  old  one  (Fig.  2,  B).  These  new  cells 
are  asexual  spores,  and  the  cell  which  has  formed  them 
within  itself  is  known  as  the  mother  cell.  This  peculiar 
kind  of  cell  division,  which  does  not  involve  the  wall  of  the 


IQ  PLANT  STRUCTURES 

old  cell,  is  often  called  internal  division,  to  distinguish  it 
from  fission,  which  involves  the  wall  of  the  old  cell,  and  is 
the  ordinary  method  of  cell  division  in  nutritive  cells. 

If  the  mother  cell  which  produces  the  spores  is  different 
from  the  other  cells  of  the  plant  body  it  is  called  the  sporan- 
gium, which  means  "  spore  vessel."  Often  a  cell  is  nutri- 
tive for  a  time  and  afterward  becomes  a  mother  cell,  in 
which  case  it  is  said  to  function  as  a  sporangium.  The  wall 
of  a  sporangium  usually  opens,  and  the  spores  are  dis- 
charged, thus  being  free  to  produce  new  plants.  Various 
names  have  been  given  to  asexual  spores  to  indicate  certain 
peculiarities.  As  Alga?  are  mostly  surrounded  by  water, 
the  characteristic  asexual  spore  in  the  group  is  one  that 
can  swim  by  means  of  minute  hair-like  processes  or  cilia, 
which  have  the  power  of  lashing  the  water  (Fig.  7,  C). 
These  ciliated  spores  are  known  as  zoospores,  or  "  animal- 
like spores,"  referring  to  their  power  of  locomotion ;  some- 
times they  are  called  sivimming  spores,  or  swarm  spores.  It 
must  be  remembered  that  all  of  these  terms  refer  to  the 
same  thing,  a  swimming  asexual  spore. 

This  method  of  reproduction  may  be  indicated  by  a  for- 
mula as  follows  :  P  —  o  —  P  —  o  —  P  —  o  —  P,  which  indi- 
cates that  new  plants  are  not  produced  directly  from  the 
old  ones,  as  in  vegetative  multiplication,  but  that  between 
the  successive  generations  there  is  the  asexual  spore. 

Sexual  spores. — These  cells  are  formed  by  cell  union, 
two  cells  fusing  together  to  form  the  spore.  This  process 
of  forming  a  spore  by  the  fusion  of  two  cells  is  called  the 
sexual  process,  and  the  two  special  cells  (sexual  cells)  thus 
used  are  known  as  gametes  (Fig.  2,  C,  d,  e).  It  must  be 
noticed  that  gametes  are  not  spores,  for  they  are  not  able 
alone  to  produce  a  new  plant ;  it  is  only  after  two  of  them 
have  fused  and  formed  a  new  cell,  the  spore,  that  a  plant 
can  be  produced.  The  spore  thus  formed  does  not  differ 
in  its  power  from  the  asexual  spore,  but  it  differs  very 
much  in  its  method  of  origin. 


THALLOPHYTES :  ALG^E  H 

The  gametes  are  organized  within  a  mother  cell,  and  if 
this  cell  is  distinct  from  the  other  cells  of  the  plant  it  is 
called  a  gametangium,  which  means  "  gamete  vessel." 

This  method  of  reproduction  may  be  indicated  by  a  for- 
mula as  follows  :  P  =  °  >  o  —  P  =  °  >  o  —  P  =  °  >  o  —  P, 
which  indicates  that  two  special  cells  (gametes)  are  pro- 
duced by  the  plant,  that  these  two  fuse  to  form  one  (sexual 
spore),  which  then  produces  a  new  plant. 

It  must  not  be  supposed  that  if  a  plant  uses  one  of  these 
three  methods  of  reproduction  (vegetative  multiplication, 
asexual  spores,  sexual  spores)  it  does  not  employ  the  other 
two.  All  three  methods  may  be  employed  by  the  same 
plant,  so  that  new  plants  may  arise  from  it  in  three  differ- 
ent ways. 


CHAPTER   III 

THE  EVOLUTION  OF  SEX 

12.  The  general  problem. — In  the  last  chapter  it  was  re- 
marked that  the  simplest  Algae  reproduce  only  by  vegetative 
multiplication,  the  ordinary  cell  division  (fission)  of  nutri- 
tive cells  multiplying  cells  and  hence  individuals.  Among 
other  low  Algae  asexual  spores  are  added  to  fission  as  a 
method  of  reproduction,  the  spores  being  also  formed  by 
cell  division,  generally  internal  division.  In  higher  forms 
gametes  appear,  and  a  new  method  of  reproduction,  the 
sexual,  is  added  to  the  other  two. 

Sexual  reproduction  is  so  important  a  process  in  all 
plants  except  the  lowest,  that  it  is  of  interest  to  discover 
how  it  may  have  originated,  and  how  it  developed  into  its 
highest  form.  Among  the  Algae  the  origin  and  develop- 
ment of  the  sexual  process  seems  to  be  plainly  suggested  ; 
and  as  all  other  plant  groups  have  probably  been  derived 
more  or  less  directly  from  Algae,  what  has  been  accom- 
plished for  the  sexual  process  in  this  lowest  group  was 
probably  done  for  the  whole  plant  kingdom. 

13.  The  origin  of  gametes. — One  of  the  best  Algae  to 
illustrate  the  possible  origin  of  gametes  is  a  common  fresh- 
water form  known  as  Ulothrix  (Fig.  2).  The  body  consists 
of  a  simple  filament  composed  of  a  single  row  of  short 
cells  (Fig.  2,  A).  Each  cell  contains  a  nucleus,  and  a 
single  large  chloroplast  which  has  the  form  of  a  thick  cyl- 
inder investing  the  rest  of  the  cell  contents.  Through  the 
microscope,  if  the  focus  is  upon  the  center  of  the  cell, 
an  optical  section  of  the  cylinder  is  obtained,  the  chloro- 

12 


THE   EVOLUTION   OF  SEX 


13 


plast  appearing  as  a  thick  green  mass  on  each  side  of  the 
central  nucleus.  As  no  other  color  appears,  it  is  evident 
that  Ulothrix  is  one  of  the  Chlorophyceae. 


A— 


Fig.  2.  Ulothrix,  a  Conferva  form.  A,  base  of  filament,  showing  lowest  holdfast 
cell  and  five  vegetative  cells,  each  with  its  single  conspicuous  cylindrical  chloro- 
plast  (seen  in  section)  inclosing  a  nucleus;  B,  four  cells  containing  numerous 
small  zoospores,  the  others  emptied;  C,  fragment  of  a  filament  showing  one  cell 
(a)  containing  four  zoospores,  another  zoospore  (b)  displaying  four  cilia  at  its 
pointed  end  and  just  having  escaped  from  its  cell,  another  cell  (<*)  from  which 
most  of  the  small  biciliate  gametes  have  escaped,  gametes  pairing  (d),  and  the 
resulting  zygotes  (e) ;  D,  beginning  of  new  filament  from  zoospore ;  E,  feeble 
filaments  formed  by  the  small  zoospores  ;  F,  zygote  growing  after  rest;  G, 
zoospores  produced  by  zygote.— Caldwell,  except  F  and  G,  which  are  after 

DODEL-PORT. 


The  cells  are  all  alike,  excepting  that  the  lowest  one  of 
the  filament  is  mostly  colorless,  and  is  elongated  and  more 
or  less  modified  to  act  as  a  holdfast,  anchoring  the  filament 
to  some  firm  support.  With  this  exception  the  cells  are  all 
nutritive  ;  but  any  one  of  them  has  the  power  of  organizing 
for  reproduction.  This  indicates  that  at  first  nutritive  and 
20 


14 


PLANT  STRUCTURES 


reproductive  cells  are  not  distinctly  differentiated,  but  that 
the  same  cell  may  be  nutritive  at  one  time  and  reproductive 
at  another. 

In  suitable  conditions  certain  cells  of  the  filament  will 
be  observed  organizing  within  themselves  new  cells  by 
internal  division  (Fig.  2,  C,  a,  b).  The  method  of  forma- 
tion at  once  suggests  that  the  new  cells  are  asexual  spores, 
and  the  mother  cell  which  produces  them  is  acting  as  a 
sporangium.  The  spores  escape  into  the  water  through  an 
opening  formed  in  the  wall  of  the  mother  cell,  and  each  is 
observed  to  have  four  cilia  at  the  pointed  end,  by  means  of 
which  it  swims,  and  hence  it  is  a  zoospore  or  swarm  spore. 
After  swimming  about  for  a  time,  the  zoospores  "  settle 
down,"  lose  their  cilia,  and  begin  to  develop  a  new  filament 
like  that  from  which  they  came  (Fig.  2,  D). 

Other  cells  of  the  same  filament  also  act  as  mother  cells, 
but  the  cells  which  they  produce  are  more  numerous,  hence 
smaller  in  size  than  the  zoospores,  and  they  have  but  two 
cilia  (Fig.  2,  C,  c).  They  also  escape  into  the  water  and 
swim  about,  except  in  size  and  in  number  of  cilia  resem- 
bling the  zoospores.  In  general  they  seem  to  be  unable  to 
act  as  the  zoospores  in  the  formation  of  new  filaments,  but 
occasionally  one  of  them  forms  a  filament  much  smaller 
than  the  ordinary  one  (Fig.  2,  E).  This  indicates  that 
they  may  be  zoospores  reduced  in  size,  and  unable  to  act  as 
the  larger  ones.  The  important  fact,  however,  is  that 
these  smaller  swimming  cells  come  together  in  pairs,  each 
pair  fusing  into  one  cell  (Fig.  2,  C,  d,  e).  The  cells  thus 
formed  have  the  power  of  producing  new  filaments  more  or 
less  directly. 

It  is  evident  that  this  is  a  sexual  act,  that  the  cell  pro- 
duced by  fusion  is  a  sexual  spore,  that  the  two  cells  which 
fuse  are  gametes,  and  that  the  mother  cell  which  produces 
them  acts  as  a  gametangium.  Cases  of  this  kind  suggest 
that  the  gametes  or  sex  cells  have  been  derived  from  zoo- 
spores, and  that  asexual  spores  have  given  rise  to  sex  cells. 


THE   EVOLUTION   OF   SEX 


15 


The  appearance  of  sex  cells  (gametes)  is  but  one  step  in  the 
evolution  of  sex.  It  represents  the  attainment  of  sexuality, 
but  the  process  becomes  much  more  highly  developed. 

14.  Isogamy. — When  gametes  first  appear,  in  some  such 
way  as  has  been  described,  the  two  which  fuse  seem  to  be 
exactly  alike.  They  resemble  each  other  in  size  and  activ- 
ity, and  in  every  structure  which  can  be  distinguished. 
This  fact  is  indicated  by  the  word  isogamy,  which  means 
"  similar  gametes,"  and  those  plants  whose  pairing  gametes 
are  similar,  as  Ulothrix,  are  said  to  be  isogamous. 

The  act  of  fusing  of  similar  gametes  is  usually  called 
conjugation,  which  means  a  "  yoking  together  "  of  similar 
bodies.  Of  course  it  is  a  sexual  process,  but  the  name  is 
convenient  as  indicating  not  merely  the  process,  but  also  an 
important  character  of  the  gametes.  The  sexual  spore 
which  results  from  this  act  of  conjugation  is  called  the 
zygote  or  zygospore,  meaning  "  yoked  spore." 

In  isogamy  it  is  evident  that  while  sexuality  has  been 
attained  there  is  no  distinction  between  sexes,  as  obtains  in 
the  higher  plants.  It  may  be  called  a  unisexual  condition, 
as  opposed  to  a  bisexual  one.  The  next  problem  in  the 
evolution  of  sex,  therefore,  is  to  discover  how  a  bisexual 
condition  has  been  derived  from  a  unisexual  or  isogamous 
one. 

15.  Heterogamy. — Beginning  with  isogamous  forms,  a 
series  of  plants  can  be  selected  illustrating  how  the  pairing 
gametes  gradually  became  unlike.  One  of  them  becomes 
less  active  and  larger,  until  finally  it  is  entirely  passive  and 
very  many  times  larger  than  its  mate  (Fig.  7).  The  other 
retains  its  small  size  and  increases  in  activity.  The  pairing 
gametes  thus  become  very  much  differentiated,  the  larger 
passive  one  being  the  female  gamete,  the  smaller  active  one 
the  male  gamete.  This  condition  is  indicated  by  the  word 
heterogamy,  which  means  "  dissimilar  gametes,"  and  those 
plants  whose  pairing  gametes  are  dissimilar  are  said  to  be 
heterogamous. 


16  PLANT  STRUCTURES 

In  order  to  distinguish  them  the  large  and  passive  female 
gamete  is  called  the  oosphere,  which  means  "  egg  sphere," 
or  it  is  called  the  egg  ;  the  small  but  active  male  gamete  is 
variously  called  the  spermatozoid,  the  antherozoid,  or  simply 
the  sperm.  In  this  book  egg  and  sperm  will  be  used,  the 
names  of  similar  structures  in  animals. 

In  isogamous  plants  the  mother  cells  (gametangia) 
which  produce  the  gametes  are  alike ;  but  in  heterogamous 
plants  the  gametes  are  so  unlike  that  the  gametangia  which 
produce  them  become  unlike.  Accordingly  they  have  re- 
ceived distinctive  names,  the  gametangium  which  produces 
the  sperms  being  called  the  antheridium,  that  producing  the 
egg  being  called  the  oogonium  (Fig.  10). 

The  act  of  fusing  of  sperm  and  egg  is  called  fertiliza- 
tion, which  is  the  common  form  of  the  sexual  process.  The 
sexual  spore  which  results  from  fertilization  is  known  as  the 
oospore  or  "  egg-spore,"  sometimes  called  the  fertilized  egg. 

It  is  evident  that  heterogamous  plants  are  bisexual,  and 
bisexuality  is  not  only  attained  among  Algae,  but  it  prevails 
among  all  higher  plants.  Among  the  lowest  forms  there  is 
only  vegetative  multiplication  ;  higher  forms  added  sexu- 
ality ;  then  still  higher  forms  became  bisexual. 

16.  Summary. — Isogamous  forms  produce  gametangia, 
which  produce  similar  gametes,  which  by  conjugation  form 
zygotes.  Heterogamous  forms  produce  antheridia  and 
oogonia,  which  produce  sperms  and  eggs,  which  by  fertiliza- 
tion form  oospores. 


CHAPTEE  IV 

THE  GREAT  GROUPS  OF  ALG^J 

17.  General  characters. — The  Algae  are  distinguished 
among  Thallophytes  by  the  presence  of  chlorophyll.  It 
was  stated  in  a  previous  chapter  that  in  three  of  the  four 
great  groups  another  coloring  matter  is  associated  with  the 
chlorophyll,  and  that  this  fact  is  made  the  basis  of  a  division 
into  Blue-green  Algae  (Cyanophyceae),  Green  Algae  (Chloro- 
phyceae),  Brown  Algae  (Phaeophyceae),  and  Eed  Algae  (Ehodo- 
phyceae).  In  our  limited  space  it  will  be  impossible  to  do 
more  than  mention  a  few  representatives  of  each  group, 
but  they  will  serve  to  illustrate  the  prominent  facts. 

1.  Cyanophycese  {Blue-green  Algce) 

18.  Glceocapsa. — These  forms  may  be  found  forming 
blue-green  or  olive-green  patches  on  damp  tree-trunks,  rock, 
walls,  etc.  By  means  of  the  microscope  these  patches  are 
seen  to  be  composed  of  multitudes  of  spherical  cells,  each 
representing  a  complete  Glceocapsa  body.  One  of  the  pecuU 
iarities  of  the  body  is  that  the  cell  wall  becomes  mucilagi- 
nous, swells,  and  forms  a  jelly-like  matrix  about  the  work- 
ing cell.  Each  cell  divides  in  the  ordinary  way,  two  new 
Glceocapsa  individuals  being  formed,  this  method  of  vegeta- 
tive multiplication  being  the  only  form  of  reproduction 
(Fig.  3). 

When  new  cells  are  formed  in  this  way  the  swollen 
mucilaginous  walls  are  apt  to  hold  them  together,  so  that 
presently  a  number  of  cells  or  individuals  are  found  lying 


18 


PLANT   STRUCTURES 


Fig.  3.  Olceocapsa,  a  blue- 
green  alga,  showing 
single  cells,  and  small 
groups  which  have  been 
formed  by  division  and 
are  held  together  by  the 
enveloping  mucilage. — 
Caldwell. 


together  imbedded  in  the  jelly-like  matrix  formed  by  the 
wall  material  (Fig.  3).  These  imbedded  groups  of  individ- 
uals are  spoken  of  as  colonies,  and  as 
colonies  become  large  they  break  up 
into  new  colonies,  the  individual  cells 
composing  them  continuing  to  divide 
and  form  new  individuals.  This  rep- 
resents a  very  simple  life  history,  in 
fact  a  simpler  one  could  hardly  be 
imagined. 

19.  Nostoc. — These  forms  occur  in 
jelly-like  masses  in  damp  places.  If 
the  jelly  be  examined  it  will  be  found 
to  contain  imbedded  in  it  numerous 
cells  like  those  of  Glceocapsa,  but  they 
are  strung  together  to  form  chains  of 
varying  lengths  (Fig.  4).  The  jelly  in 
which  these  chains  are  imbedded  is  the 
same  as  that  found  in  Glceocapsa,  being 
formed  by  the  cell  walls  becoming  mucilaginous  and  swollen. 
One  notable  fact  is  that  all  the  cells  in  the  chain  are  not 
alike,  for  at  irregu- 
lar intervals  there  oc- 
cur larger  colorless 
cells,  an  illustration 
of  the  differentiation 
of  cells.  These  larger 
cells  are  known  as  het- 
erocysts  (Fig.  4,  A), 
which  simply  means 
"other  cells."  It  is 
observed  that  when 
the  chain  breaks  up 

into    fragments   each        pTG.   4.     Nostoc,  a  bine-green  alga,  showing  the 

fragment  is  COmT)OSed  chain-like   filaments,  and    the    heterocysts   (4) 

„     ,  n      i  which  determine  the  breaking  up  of  the  chain 

oi  the  cells  between         -Caldwell. 


THE   GREAT   GROUPS   OF   ALG/E 


19 


two  heterocysts.  The  fragments  wriggle  out  of  the  jelly 
matrix  and  start  new  colonies  of  chains,  each  cell  dividing 
to  increase  the  length  of  the  chain.  This  cell  division, 
to  form  new  cells,  is  the  characteristic  method  of  repro- 
duction. 

At  the  approach  of  unfavorable  conditions  certain  cells 
of  the  chain  become  thick-walled  and  well-protected.  These 
cells  which  endure  the  cold  or  other  hardships,  and  upon 
the  return  of  favorable  conditions  produce  new  chains  of 
3ells,  are  often  called  spores,  but  they  are  better  called 
1  resting  cells." 

20.  Oscillatoria. — These  forms  are  found  as  bluish-green 
slippery  masses  on  wet  rocks,  or  on  damp  soil,  or  freely 
floating.     They  are  simple  filaments,  composed  of  very  short 
flattened  cells  (Fig.  5),  and  the  name 
Oscillatoria  refers  to  the  fact  that  they 
exhibit  a  peculiar  oscillating  move- 
ment.   These  motile  filaments  are  iso- 
lated, not  being  held  together  in  a 
jelly-like  matrix  as  are  the  chains  of 
Nostoc,  but  the  wall  develops  a  cer- 
tain amount  of  mucilage,  which  gives 
the  slippery  feeliDg  and    sometimes 
forms    a    thin    mucilaginous    sheath 
about  the  row  of  cells. 

The  cells  of  a  filament  are  all  alike, 
except  that  the  terminal  cell  has  its 
free  surface  rounded.  If  a  filament 
breaks  and  a  new  cell  surface  ex- 
posed, it  at  once  becomes  rounded. 
If  a  single  cell  of  the  filament  is 
freed  from  all  the  rest,  both  flattened  ends  become  rounded, 
and  the  cell  becomes  spherical  or  nearly  so.  These  facts 
indicate  at  least  two  important  things  :  (1)  that  the  cell 
wall  is  elastic,  so  that  it  can  be  made  to  change  its  form, 
and  (2)  that  it  is  pressed  upon  from  within,  so  that  if  free 


Fia.  5.  Oscillatoria,  a 
blue-green  alga,  show- 
ing a  group  of  filaments 
{A),  and  a  single  fila- 
ment more  enlarged  (B). 
— Caldwell. 


20  PLANT  STRUCTURES 

it  will  bulge  outward.     In  all  active  living  cells  there  is 
this  pressure  upon  the  wall  from  within. 

Each  cell  of  the  Oscillatoria  filament  has  the  power  of 
dividing,  thus  forming  new  cells  and  elongating  the  fila- 
ment. A  filament  may  break  up  into  fragments  of  varying 
lengths,  and  each  fragment  by  cell  division  organizes  a  new 
filament.  Here  again  reproduction  is  by  means  of  vegeta- 
tive multiplication. 

21.  Conclusions. — Taking  Gloeocapsa,  Nostoc,  and  Oscil- 
latoria as  representatives  of  the  group  Cyanophycese,  or 
"  green  slimes,"  we  may  come  to  some  conclusions  concern- 
ing the  group  in  general.  The  plant  body  is  very  simple, 
consisting  of  single  cells,  or  chains  and  filaments  of  cells. 
Although  in  Nostoc  and  Oscillatoria  the  cells  are  organized 
into  chains  and  filaments,  each  cell  seems  to  be  able  to  live 
and  act  independently,  and  the  chain  and  filament  seem  to 
be  little  more  than  colonies  of  individual  cells.  In  this 
sense,  all  of  these  plants  may  be  regarded  as  one-celled. 

Differentiation  is  exhibited  in  the  appearance  of  hetero- 
cysts  in  Nostoc,  peculiar  cells  which  seem  to  be  connected 
in  some  way  with  the  breaking  up  of  filamentous  colonies, 
although  the  Oscillatoria  filament  breaks  up  without  them. 

The  power  of  motion  is  also  well  exhibited  by  the  group, 
the  free  filaments  of  Oscillatoria  moving  almost  continu- 
ally, and  the  imbedded  chains  of  Nostoc  at  times  moving  to 
escape  from  the  restraining  mucilage. 

The  whole  group  also  shows  a  strong  tendency  in  the 
cell-wall  material  to  become  converted  into  mucilage  and 
much  swollen,  a  tendency  which  reaches  an  extreme  expres- 
sion in  such  forms  as  Nostoc  and  Gloeocapsa. 

Another  distinguishing  mark  is  that  reproduction  is 
exclusively  by  means  of  vegetative  multiplication,  through 
ordinary  cell  division  or  fission,  which  takes  place  very 
freely.  Individual  cells  are  organized  with  heavy  resistant 
walls  to  enable  them  to  endure  the  winter  or  other  unfavor- 
able conditions,  and  to  start  a  new  series  of  individuals 


THE  GREAT  GEOUPS  OF  ALGJS 


21 


upon  the  return  of  favorable  conditions.  These  may  be 
regarded  as  resting  cells.  So  notable  is  the  fact  of  repro- 
duction by  fission  that  Cyanophycese  are  often  separated 
from  the  other  groups  of  Alga?  and  spoken  of  as  "  Fission 
Algae,"  which  put  in  technical  form  becomes  Schizophyceae. 
In  this  particular,  and  in  several  others  mentioned  above, 
they  resemble  the  "  Fission  Fungi  "  (Schizomycetes),  com- 
monly called  "bacteria,"  so  closely  that  they  are  often 
associated  with  them  in  a  common  group  called  "  Fis- 
sion plants "  (Schizophytes),  distinct  from  the  ordinary 
Algae  and  Fungi. 


2.  Chlorophyce^;  (Green  Algce). 

22.  Pleurococcus. — This  may  be  taken  as  a  type  of  one- 
celled  Green  Algae.  It  is  most  commonly  found  in  masses 
covering  damp  tree-trunks,  etc.,  and  looking  like  a  green 
stain.  These  fine- 
ly granular  green 
masses  are  found 
to  be  made  up 
of  multitudes  of 
spherical  cells  re- 
sembling those  of 
Gloeocapsa,  except 
that  there  is  no 
blue  with  the  chlo- 
rophyll, and  the 
cells  are  not  im- 
bedded in  such 
jelly-like  masses. 
The  cells  may  be 
solitary,  or  may 
cling  together  in 

colonies  of  various  sizes  (Fig.  6).     Like  Glmocapsa,  a  cell 
divides  and  forms  two  new  cells,  the   only  reproduction 


Pig.  6.  Pleurococcus,  a  one-celled  green  alga  :  A,  show- 
ing the  adnlt  form  with  its  nucleus  ;  B,  C,  D,  E, 
various  stages  of  division  (fission)  in  producing  new 
cells  ;  F,  colonies  of  cells  which  have  remained  in 
contact. —C  ALDWELL. 


22  PLANT   STRUCTURES 

being  of  this  simple  kind.  It  is  evident,  therefore,  that  the 
group  Chlorophyceae  begins  with  forms  just  as  simple  as 
are  to  be  found  among  the  Cyanophyceas. 

Pleurococvus  is  used  to  represent  the  group  of  Protococ- 
cus  forms,  one-celled  forms  which  constitute  one  of  the 
subdivisions  of  the  Green  Algge.  It  should  be  said  that 
Pleurococcus  is  possibly  not  a  Protococcus  form,  but  may 
be  a  reduced  member  of  some  higher  group ;  but  it  is  so 
common,  and  represents  so  well  a  typical  one-celled  green 
alga,  that  it  is  used  in  this  connection.  It  should  be 
known,  also,  that  while  the  simplest  Protococcus  forms  re- 
produce only  by  fission,  others  add  to  this  the  other  meth- 
ods of  reproduction. 

23.  Ulothrix. — This  form  was  described  in  §  13.  It 
is  very  common  in  fresh  waters,  being  recognized  easily  by 
its  simple  filaments  composed  of  short  squarish  cells,  each 
cell  containing  a  single  conspicuous  cylindrical  chloroplast 
(Fig.  2).  This  plant  uses  cell  division  to  multiply  the  cells 
of  a  filament,  and  to  develop  new  filaments  from  fragments 
of  old  ones ;  but  it  also  produces  asexual  spores  in  the  form 
of  zoospores,  and  gametes  which  conjugate  and  form  zygotes. 
Both  zoospores  and  zygotes  have  the  power  of  germination — 
that  is,  the  power  to  begin  the  development  of  a  new  plant. 
In  the  germination  of  the  zygote  a  new  filament  is  not  pro- 
duced directly,  but  there  are  formed  within  it  zoospores, 
each  of  which  produces  a  new  filament  (Fig.  2,  F,  G).  All 
three  kinds  of  reproduction  are  represented,  therefore,  but 
the  sexual  method  is  the  low  type  called  isogamy,  the  pair- 
ing gametes  being  alike. 

Ulothrix  is  taken  as  a  representative  of  the  Conferva 
forms,  the  most  characteristic  group  of  Chlorophyceae. 
All  the  Conferva  forms,  however,  are  not  isogamous,  as  will 
be  illustrated  by  the  next  example. 

24t.  (Edogonium. — This  is  a  very  common  green  alga, 
found  in  fresh  waters  (Fig.  7).  The  filaments  are  long  and 
simple,  the  lowest  cell  acting  as  a  holdfast,  as  in  Ulothrix 


Wmm 

\m 

i    Sff  '4i 

w\ 

iJk  i 

Pj 

i*      t .' 

m 

Fig.  7.  (Edogonium  nodosum,  a  Conferva  form  :  A,  portion  of  a  filament  showing  a 
vegetative  cell  with  its  nucleus  (dh  an  oogonium  (a)  filled  by  an  egg  packed  with 
food  material,  a  second  oogonium  (c)  containing  a  fertilized  egg  or  oospore  as 
shown  by  the  heavy  wall,  and  two  antheridia  (ft),  each  containing  two  sperms;  B, 
another  filament  snowing  antheridia  (a)  from  which  two  sperms  (b)  have  escaped, 
a  vegetative  cell  with  its  nucleus,  and  an  oogonium  which  a  sperm  (e)  has  entered 
and  Is  coming  in  contact  with  the  egg  whose  nucleus  (d)  may  be  seen;  C,  a  zoo- 
spore which  has  been  formed  in  a  vegetative  cell,  showing  the  crown  of  cilia  and 
the  clear  apex,  as  in  the  sperms;  J),  a  zoospore  producing  a  new  filament,  putting 
ont  a  holdfast  at  base  and  elongating;  E,  a  further  stage  of  development;  F.  the 
four  zoospores  formed  by  the  oospore  when  it  germinates  -Caldwell,  except 
Cand  F,  which  are  after  Pringshbim. 


24  PLANT   STRUCTURES 

(§13).  The  other  cells  are  longer  than  in  Ulothrix,  each 
cell  containing  a  single  nucleus  and  apparently  several 
chloroplasts,  but  really  there  is  but  one  large  complex 
chloroplast. 

The  cells  of  the  filament  have  the  power  of  division, 
thus  increasing  the  length  of  the  filament.  Any  cell  also 
may  act  as  a  sporangium,  the  contents  of  a  mother  cell 
organizing  a  single  large  asexual  spore,  which  is  a  zoospore. 
The  zoospore  escapes  from  the  mother  cell  into  the  water, 
and  at  its  more  pointed  clear  end  there  is  a  little  crown  of 
cilia,  by  means  of  which  it  swims  about  rapidly  (Fig.  7,  0). 
After  moving  about  for  a  time  the  zoospore  comes  to  rest, 
attaches  itself  by  its  clear  end  to  some  support,  elongates, 
begins  to  divide,  and  develops  a  new  filament  (Fig.  7,  D,  E). 

Other  cells  of  the  filament  become  very  different  from 
the  ordinary  cells,  swelling  out  into  globular  form  (Fig.  7, 
A,  B),  and  each  such  cell  organizes  within  itself  a  single 
large  egg  (oosphere).  As  the  egg  is  a  female  gamete,  the 
large  globular  cell  which  produces  it,  and  which  is  differen- 
tiated from  the  other  cells  of  the  body,  is  the  oogonium. 
A  perforation  in  the  oogonium  wall  is  formed  for  the 
entrance  of  sperms. 

Other  cells  in  the  same  filament,  or  in  some  other  fila- 
ment, are  observed  to  differ  from  the  ordinary  cells  in 
being  much  shorter,  as  though  an  ordinary  cell  had  been 
divided  several  times  without  subsequent  elongation  (Fig. 
7,  A,  /,  B,  a).  In  each  of  these  short  cells  one  or  two 
sperms  are  organized,  and  therefore  each  short  cell  is  an 
antheridium.  When  the  sperms  are  set  free  they  are  seen 
to  resemble  very  small  zoospores,  having  the  same  little 
crown  of  cilia  at  one  end. 

The  sperms  swim  actively  about  in  the  vicinity  of  the 
oogonia,  and  sooner  or  later  one  enters  the  oogonium 
through  the  perforation  provided  in  the  wall,  and  fuses 
with  the  egg  (Fig.  7,  B,  c).  As  a  result  of  this  act  of  fer- 
tilization an  oospore  is  formed,  which  organizes  a  firm  wall 


THE   GREAT   GROUPS   OF   ALG.E 


25 


about  itself.  This  firm  wall  indicates  that  the  oospore  is 
not  to  germinate  immediately,  but  is  to  pass  into  a  resting 
condition.  Spores  which  form  heavy  walls  and  pass  into 
the  resting  con- 
dition are  often 
spoken  of  as  "  rest- 
ing spores,"  and  it 
is  very  common 
for  the  zygotes 
and  oospores  to 
be  resting  spores. 
These  resting 
spores  enable  the 
plant  to  endure 
through  unfavor- 
able conditions, 
such  as  failure  of 
food  supply,  cold, 
drought,  etc. 
When  favorable 
conditions  return, 
the  protected  rest- 
ing spore  is  ready 
for  germination. 

When     the 
oospore  of  (Edogo- 


Pio.  8.  Cladophora,  a  branching  green  alga,  a  very 
small  part  of  the  plant  being  shown.  The  branches 
arise  at  the  upper  ends  of  cells,  and  the  cells  are 
ccenocytic.— Caldwbli.. 


nium   germinates 

it  does  not  develop  directly  into  a  new  filament,  but  the 
contents  become  organized  into  four  zoospores  (Fig.  7,  F), 
which  escape,  and  each  zoospore  develops  a  filament.  In 
this  way  each  oospore  may  give  rise  to  four  filaments. 

It  is  evident  that  (Edogonium  is  a  heterogamous  plant, 
and  is  another  one  of  the  Conferva  forms.  Conferva  bodies 
are  not  always  simple  filaments,  as  are  those  of  Ulothrix 
and  (Edogonium,  but  they  are  sometimes  extensively  branch- 
ing filaments,  as  in  Cladophora,  a  green  alga  very  common 


26 


PLANT  STRUCTURES 


in  rivers  and  lakes  (Fig.  8).  The  cells  are  long  and  densely 
crowded  with  chloroplasts  ;  and  in  certain  cells  at  the  tips 
of  branches  large  numbers  of  zoospores  are  formed,  which 
have  two  cilia  at  the  pointed  end,  and  hence  are  said  to  be 
biciliate. 

25.  Vaucheria. — This  is  one  of  the  most  common  of  the 
Green  Algae,  found  in  felt-like  masses  of  coarse  filaments  in 
shallow  water  and  on  muddy  banks,  and  often  called  "  green 


Pig.  9.  Vaucheria  geminata,  a  Siphon  form,  showing  a  portion  of  the  coenocytic 
body  {A)  which  has  sent  out  a  branch  at  the  tip  of  which  a  sporangium  (B) 
formed,  within  which  a  large  zoospore  was  organized,  and  from  which  (Z>)  it  is 
discharged  later  as  a  large  multiciliate  body  ( C),  which  then  begins  the  develop- 
ment of  a  new  coenocytic  body  (E ).— Caldwell. 


felt."  The  filament  is  very  long,  and  usually  branches  ex- 
tensively, but  its  great  peculiarity  is  that  there  is  no  parti- 
tion wall  in  the  whole  body,  which  forms  one  long  continuous 
cavity  (Figs.  9,  11).  This  is  sometimes  spoken  of  as  a  one- 
celled  body,  but  it  is  a  mistake.  Imbedded  in  the  exten- 
sive cytoplasm  mass,  which  fills  the  whole  cavity,  there  are 
not  only  very  numerous  chloroplasts,  but  also  numerous 
nuclei.     As  has  been  said,  a  single  nucleus  with  some  cyto- 


THE  GREAT  GROUPS  OF  ALG^E  27 

plasm  organized  about  it  is  a  cell,  whether  it  has  a  wall  or 
not.  Therefore  the  body  of  Vaucheria  is  made  up  of  as 
many  cells  as  there  are  nuclei,  cells  whose  protoplasmic 
structures  have  not  been  kept  separate  by  cell  walls.  Such 
a  body,  made  up  of  numerous  cells,  but  with  no  partitions, 
is  called  a  camocyte,  or  it  is  said  to  be  ccenocytic.  Vaucheria 
represents  a  great  group  of  Chlorophyceae  whose  members 
have  ccenocytic  bodies,  and  on  this  account  they  are  called 
the  Siphon  forms. 

Vaucheria  produces  very  large  zoospores.  The  tip  of  a 
branch  becomes  separated  from  the  rest  of  the  body  by  a 
partition  and  thus  acts  as  a  sporangium  (Fig.  9,  B).  In 
this  improvised  sporangium  the  whole  of  the  contents  or- 
ganize a  single  large  zoospore,  which  is  ciliated  all  over, 
escapes  by  squeezing  through  a  perforation  in  the  wall 
(Fig.  9,  (7),  swims  about  for  a  time,  and  finally 
develops  another  Vaucheria  body  (Figs.  9,  £J,  10). 
It  should  be  said  that  this  large  body,  called 
a  zoospore  and  acting  like  one,  is  really  a 
mass  of  small  biciliate  zoospores,  just  as  the 


Fig.  10.  A  young  Vaucheria  germinating  from  a 
spore  (sp),  and  showing  the  holdfast  (w).— 
After  Sachs. 

apparently  one-celled  vegetative  body  is  really  composed  of 
many  cells.  In  this  large  compound  zoospore  there  are 
many  nuclei,  and  in  connection  with  each  nucleus  two  cilia 
are  developed.  Each  nucleus  with  its  cytoplasm  and  two 
cilia  represents  a  small  biciliate  zoospore,  such  as  those  of 
Cladophora,  §  24. 

Antheridia  and  oogonia  are  also  developed.  In  a  com- 
mon form  these  two  sex  organs  appear  as  short  special 
branches  developed  on  the  side  of  the  large  ccenocytic  body, 


28 


PLANT  STRUCTURES 


and  cut  off  from  the  general  cavity  by  partition  walls  (Fig. 
11).     The  oogonium  becomes  a  globular  cell,  which  usually 


Fig.  11.  Yaucheria  sesgilis,  a  Siphon  form,  show- 
ing a  portion  of  the  ccenocytic  body,  an  an- 
theridial  branch  (A)  with  an  empty  anthe- 
ridium  (a)  at  its  tip,  and  an  oogonium  (B) 
containing  an  oospore  (c)  and  showing  the 
opening  (/)  through  which  the  sperms  passed 
to  reach  the  egg.— Caldwell. 


develops  a  perforated  beak  for 
the  entrance  of  the  sperms,  and 
organizes  within  itself  a  single 
large  egg  (Fig.  11,  B).  The  an- 
theridium  is  a  much  smaller  cell, 
within  which  numerous  very  small 
sperms  are  formed  (Fig.  11,  A,  a). 
The  sperms  are  discharged,  swarm 
about  the  oogonium,  and  finally 
one  passes  through  the  beak  and 
fuses  with  the  egg,  the  result  be- 
ing an  oospore.  The  oospore  or- 
ganizes a  thick  wall  and  becomes 
a  resting  spore. 

It  is  evident  that  Vaucheria  is  heterogamous,  but  all  the 
other  Siphon  forms  are  isogamous,  of  which  Botrydium  may 
be  taken  as  an  illustration  (Fig.  12). 

26.  Spirogyra. — This  is  one  of  the  commonest  of  the 
"pond  scums,"  occurring  in  slippery  and  often  frothy 
masses  of  delicate  filaments  floating  in  still  water  or  about 


Fig.  12.  Botrydium,  one  of  the 
Siphon  forms  of  green  algae, 
the  whole  body  containing 
one  continuous  cavity,  with 
a  bulbous,  chlorophyll-con- 
taining portion,  and  root- 
like branches  which  pene- 
trate the  mud  in  which 
the  plant  grows. — Cald- 
well. 


THE  GREAT  GRODPS  OF  ALGM 


29 


springs.     The  filaments  are  simple,  and  are  not  anchored 
by  a  special  basal  cell,  as  in  Ulothrix  and  (Edogonitwi.     The 


Fig.  13.  Spirogyra,  a  Conjugate  form,  showing  one  complete  cell  and  portions  of 
two  others.  The  band-like  chloroplasts  extend  in  a  spiral  from  one  end  of  the 
cell  to  the  other,  in  them  are  imbedded  nodule-like  bodies  (pyrenoids),  and  near 
the  center  of  the  cell  the  nucleus  is  swung  by  radiating  strands  of  cytoplasm.— 
Caldwell. 

cells  contain  remarkable  chloroplasts,  which  are  bands  pass- 
ing spirally  about  within  the  cell  wall.     These  bands  may 


Pig.  14.    Spirogyra,  showing  conjugation :  A,  conjugating  tubes  approaching  each 
other;  B,  tubes  in  contact  but  end  walls  not  absorbed:  C,  tube  complete  and  con- 
tents of  one  cell  passing  through;  D,  a  completed  zygospore.— Caldwell. 
21 


30 


PLANT  STRUCT URES 


be  solitary  or  several  in  a  cell,  and  form  very  striking  and 
conspicuous  objects  (Figs.  13,  14). 

Spirogyra  and  its  associates  are  further  peculiar  in  pro- 
ducing no  asexual  spores,  and  also  in  the  method  of  sexual 
reproduction.  Two  adjacent  filaments  put  out  tubular 
processes  toward  one  another.  A  cell  of  one  filament  sends 
out  a  process  which  seeks  to  meet  a  corresponding  process 
from  a  cell  of  the  other  filament.  When  the  tips  of  two 
such   processes  come   together,  the   end   walls   disappear, 


Fig.  15.  Spirogyra,  showing  some  common  exceptions.  At  A  two  cells  have  been 
connected  by  a  tube,  but  without  fusion  a  zygote  has  been  organized  in  each  cell; 
also,  the  upper  cell  to  the  left  has  attempted  to  conjugate  with  the  cell  to  the 
right.  At  B  there  are  cells  from  three  filaments,  the  cells  of  the  central  one  hav- 
ing conjugated  with  both  of  the  others.— Caldwell. 


and  a  continuous  tube  extending  between  the  two  cells  is 
organized  (Figs.  14,  15).  When  many  of  the  cells  of  two 
parallel  filaments  become  thus  united,  the  appearance  is 
that  of  a  ladder,  with  the  filaments  as  the  side  pieces,  and 
the  connecting  tubes  as  the  rounds. 

While  the  connecting  tube  is  being  developed  the  con- 
tents of  the  two  cells  are  organizing,  and  after  the  comple- 
tion of  the  tube  the  contents  of  one  cell  pass  through  and 
enter  the  other  cell,  fuse  with  its  contents,  and  a  sexual 


THE   GREAT   GROUPS   OF  ALGiE 


31 


spore  is  organized.  As  the  gametes 
look  alike,  the  process  is  conjuga- 
tion, and  the  sex  spore  is  a  zygote, 
which,  with  its  heavy  wall,  is  rec- 
ognized to  be  a  resting  spore.  At 
the  beginning  of  each  growing 
season,  the  well-protected  zygotes 
which  have  endured  the  winter 
germinate  directly  into  new  Spi- 
rogyra  filaments. 

On  account  of  this  peculiar 
style  of  sexual  reproduction,  in 
which  gametes  are  not  discharged, 
but  reach  each  other  through  spe- 
cial tubes,  Spirogyra  and  its  allies 
are  called  Conjugate  forms — that 
is,  forms  whose  bodies  are  "  yoked 
together  "  during  the  fusion  of  the 
gametes. 

In  some  of  the  Conjugate  forms 
the  zygote  is  formed  in  the  connect- 
ing tube  (Fig.  16,  A),  and  some- 
times zygotes  are  formed  without 
conjugation  (Fig.  16,  B).  Among 
the  Conjugate  forms  the  Desmids 
are  of  great  interest  and  beauty, 
being  one-celled,  the  cells  being 
organized  into  two  distinct  halves 
(Fig.  17). 

27.  Conclusions.  —  The  Green 
Alga?,  as  indicated  by  the  illustra- 
tions given  above,  include  simple 
one-celled  forms  which  reproduce 
by  fission,  but  they  are  chiefly  fila- 
mentous forms,  simple  or  branching.  These  filamentous 
bodies  either  have  the  cells  separated  from  one  another 


Fig.  16.  Two  Conjugate  forms  : 
A  (Mmigeotia),  showing  for- 
mation of  zygote  in  conjuga- 
ting tube  ;  B,  C  (Gonatone- 
ma),  showing  formation  of 
zygote  without  conjugation. 
— After  Wittrock. 


32 


PLANT  STRUCTURES 


by  walls,  or  they  are  ccenocytic,  as  in  the  Siphon  forms. 
The  characteristic  asexual  spores  are  zoospores,  but  these 
may  be  wanting,  as  in  the  Conjugate  forms.  In  addition 
to  asexual  reproduction,  both  isogamy  and  heterogamy  are 
developed,  and  both  zygotes  and  oospores  are  resting  spores. 


Fig.  17.    A  group  of  Desmids,  one-celled  Conjugate  forms,  showing  various  pat- 
terns, and  the  cells  organized  into  distinct  halves. — After  Kerner. 

The  Green  Algae  are  of  special  interest  in  connection 
with  the  evolution  of  higher  plants,  which  are  supposed  by 
some  to  have  been  derived  from  them. 


3.  Ph^ophyce^  (Brown  Algm) 

28.  General  characters. — The  Blue-green  Algae  and  the 
Green  Algae  are  characteristic  of  fresh  water,  but  the  Brown 
Algae,  or  "  kelps,"  are  almost  all  marine,  being  very  charac- 


THE   GREAT   GROUPS   OF  ALG^E 


33 


teristic  coast  forms.    All  of  them  are  anchored  by  holdfasts, 
which  are  sometimes  highly  developed  root-like  structures ; 
and  the  yellow,  brown,  or  olive-green  floating 
bodies  are  buoyed  in  the  water  usually  by  the 
aid  of  floats   or  air-bladders,  which   are  often 
very  conspicuous.     The  kelps  are  most  highly 
developed  in  the  colder  waters,  and  form  much 
of  the  "  wrack,"  "  tangle,"  etc.,  of  the  coasts. 
The   group   is  well  adapted   to 
live  exposed  to  waves  and  cur- 
rents with  its  strong  holdfasts, 
air-bladders,  and  tough  leathery 
bodies.     Certain  Brown  Algae,  as 
Ectocarpus  (Fig.  18), are  of  great 
interest  on  account  of  their  pos- 
sible relation  to  the  evolution  of 
higher  plants.    It  is 
in  this  group  that  we 
have  found  our  only 
suggestions  as  to  the 
origin  of  the  complex 
sex-organs  occurring 
in    Bryophytes    and 
Pteridophytes. 

29.  The  plant 
body. — There  is  very 
great  diversity  in  the 
structure  of  the 
plant  body.  Some 
of  them,  as  Ed oca  r- 
pus  (Fig.  18),  are  fil- 
amentous forms,  like 
the  Confervas  among 
the  Green  Algae,  but 
others  are  very  much  more  complex.  The  thallus  of  Lam- 
inaria  is  like  a  huge  floating  leaf,  frequently  nine  to  ten 


Fig.  18.  A  brown  alga  (Ectocarpus),  showing  a 
body  consisting  of  a  simple  filament  which  puts 
out  branches  (A),  some  sporangia  (B)  contain- 
ing zoospores,  and  gametangia  (C)  containing 
gametes.— Caldwell. 


ess 


Fio.  18a.    A  group  of  brown  seaweeds  (Laminarias).    Note  the  various  habits  of 
the  plant  body  with  its  leaf -like  thallus  and  root-like  holdfasts. -After  Keener. 


THE   GREAT   GROUPS   OF  ALG^E 


35 


feet  long,  whose  stalk  develops  root-like  holdfasts  (Fig.  18a). 
The  largest  body  is  developed  by  an  Antarctic  Laminaria 
form,  which  rises  to  the  surface  from  a  sloping  bottom  with 
a  floating  thallus  six  hundred  to  nine  hundred  feet  long. 
Other  forms  rise  from  the  sea  bottom  like  trees,  with 
thick  trunks,  numerous  branches,  and  leaf-like  appendages. 

The  common  Fucus, 
or  "  rock  weed,"  is  rib- 
bon-form and  constantly 
branches  by  forking  at 
the  tip  (Fig.  19).  This 
method  of  branching  is 
called  dichotomous,  as  dis- 
tinct from  that  in  which 
branches  are  put  out 
from  the  sides  of  the  axis 
{monopodial).  The  swol- 
len air-bladders  distrib- 
uted throughout  the  body 
are  very  conspicuous. 

The  most  differenti- 
ated thallus  is  that  of 
Sargassum  (Fig.  20),  or 
"  gulf  weed,"  in  which 
there  are  slender  branch- 
ing stem-like  axes  bearing 
lateral  members  of  various 
kinds,  some  of  them  like 
ordinary  foliage  leaves ; 
others  are  floats  or  air- 
bladders,  which  sometimes 

resemble  clusters  of  berries ;  and  other  branches  bear  the 
sex  organs.  All  of  these  structures  are  but  different  regions 
of  a  branching  thallus.  Sargassum  forms  are  often  torn 
from  their  anchorage  by  the  waves  and  carried  away  from 
the  coast  by  currents,  collecting  in  the  great  sea  eddies 


Fig.  19.  Fragment  of  a  common  brown 
alga  (Fucus),  showing  the  body  with 
dichotomous  branching  and  bladder-like 
air-bladders.— After  Luerssen. 


36 


PLANT  STRUCTURES 


produced  by  oceanic  currents  and  forming  the   so-called 
"  Sargasso  seas,"  as  that  of  the  North  Atlantic. 


Fig.  20.  A  portion  of  a  brown  alga  (Sargassum),  showing  the  thallus  differentiated 
into  stem-like  and  leaf-like  portions,  and  also  the  bladder-like  floats.— After  Ben- 
nett and  Mukkay. 


30.  Reproduction. — The  two  main  groups  of  Brown  Algae 
differ  from  each  other  in  their  reproduction.  One,  repre- 
sented by  the  Laminarias  aud  a  majority  of  the  forms,  pro- 
duces zoospores  and  is  isogamous  (Fig.  18).  The  zoospores 
and  gametes  are  peculiar  in  having  the  two  cilia  attached 
at  one  side  rather  than  at  an  end ;  and  they  resemble  each 
other  very  closely,  except  that  the  gametes  fuse  in  pairs  and 
form  zygotes. 


a 


1,V.         ^ jr 


F         \ 


S 


V* 


fcV> 


Pig.  21.  Sexual  reproduction  of  Fswaw,  showing  the  eight  eggs  (six  in  sight)  dis- 
charged from  the  oogonium  and  surrounded  by  a  membrane  (.4),  eggs  liberated 
from  the  membrane  (E),  antheridinm  containing  sperms  (  C),  the  discharged  lat- 
erally biciliate  sperms  Iff),  and  eggs  surrounded  by  swarming  sperms  (F,  H ).— 
After  Singer. 


38 


PLANT    STRUCTUKES 


The  other  group,  represented  by  Fucus  (Fig.  21),  pro- 
duces  no  asexual  spores,  but  is  heterogamous.  A  single 
oogonium  usually  forms  eight  eggs  (Fig.  21,  A),  which  are 
discharged  and  float  freely  in  the  water  (Fig.  21,  E).  The 
antheridia  (Fig.  21,  C)  produce  numerous  minute  laterally 
biciliate  sperms,  which  are  discharged  (Fig.  21,  G),  swim  in 
great  numbers  about  the  large  eggs  (Fig.  21,  F,  H),  and 
finally  one  fuses  with  an  egg,  and  an  oospore  is  formed. 
As  the  sperms  swarm  very  actively  about  the  egg  and 
impinge  against  it  they  often  set  it  rotating.  Both  an- 
theridia and  oogonia  are  formed  in  cavities  of  the  thallus. 

4.  Khodophyce^:  (Red  Algce) 
31.  General  characters. — On  account  of  their  red  colora- 
tion these  forms  are  often  called  Floridece.    They  are  mostly 

marine  forms,  and  are 
anchored  by  holdfasts 
of  various  kinds.  They 
belong  to  the  deepest 
waters  in  which  Algae 
grow,  and  it  is  probable 
that  the  red  coloring 
matter  which  character- 
izes them  is  associated 
with  the  depth  at  which 
they  live.  The  Red 
Algae  are  also  a  high- 
ly specialized  line,  and 
will  be  mentioned  very 
briefly.. 

32.   The  plant  body. 
—  The    Eed    Algae,    in 
general,  are  more  deli- 
cate   than    the    Brown 
Algae,  or  kelps,  their  graceful  forms,  delicate  texture,  and 
brightly  tinted  bodies  (shades  of  red,  violet,  dark  purple, 


^ig.  22.  A  red  alga  (Gigartina),  showing 
branching  habit,  and  "fruit  bodies." — 
After  Schenck. 


Fig.  34.    A  red  alga  (Dasya),  showing  a  finely  divided  thallus  body. 
Caldwell. 


Fig.  25.    A  red  alga  (Rabdonia),  showing  holdfasts  and  branching  thallus  body.— 

Caldwell. 


Fig.  20.    A  red  alga  (Plilota),  whose  branching  body  resembles  moss. — 
Caldwell. 


THE   GREAT   GROUPS   OF  ALG^E 


43 


and  reddish-brown)  making  them  very  attractive.  They 
show  the  greatest  variety  of  forms,  branching  filaments, 
ribbons,  and  filmy  plates  prevailing,  sometimes  branching 
very  profusely  and  delicately,  and  resembling  mosses  of 
fine  texture  (Figs.  22,  23,  24,  25,  26).  The  differentiation 
of  the  thallus  into  root  and  stem  and  leaf-like  structures 
is  also  common,  as  in  the  Brown  Algae. 

33.  Reproduction. — Red  Algae  are  very  peculiar  in  both 
their  asexual  and  sexual  reproduction.  A  sporangium  pro- 
duces just  four  asexual  spores,  but  they  have  no  cilia  and 
no  power  of  motion.  They 
can  not  be  called  zoospores, 
therefore,   and  as    each    spo- 


Fig.  27.  A  red  alga  (Callilhamnion),  show- 
ing sporangium  (A),  and  the  tetraspores 
discharged  (B).—  After  Thurkt. 


Fig.  28.  A  red  alga  (Nemaliori) ;  A, 
sexual  branches,  showing  antheri- 
dia<a),  oogonium  (o)  with  its  trich- 
ogyne  (t),  to  which  are  attached  two 
spennatia  (g);  B,  beginning  of  a 
cystocarp  (o),  the  trichogyne  (I)  still 
showing;  C,  an  almost  mature  cys- 
tocarp (o),  with  the  disorganizing 
trichogyne  (t). — After  Knt. 


rangium  always  produces  just 
four,  they  have  been  called 
tetraspores  (Fig.  27). 

Red  Algas  are  also  heterog- 
amous,  but  the  sexual  process  has  been  so  much  and  so 
variously  modified  that  it  is  very  poorly  understood.  The 
antheridia  (Fig.  28,  A,  a)  develop  sperms  which,  like  the 
tetraspores,  have  no  cilia  and  no  power  of  motion.     To  dis- 


44 


PLANT  STRUCTURES 


B 


?0^ 


tinguish  them  from  the  ciliated  sperms,  or  spermatozoids, 
which  have  the  power  of  locomotion,  these  motionless  male 
gametes  of  the  Eed  Algae  are  usually  called  spermatid 
(singular,  spermatium)  (Fig.  28,  A,  s). 

The  oogonium  is  very  pe- 
culiar, being  differentiated 
into  two  regions,  a  bulbous 
base  and  a  hair-like  process 
(trichogyne),  the  whole  struc- 
ture resembling  a  flask  with  a 
long,  narrow  neck,  excepting 
that  it  is  closed  (Fig.  28,  A, 
o,  t).  Within  the  bulbous  part 
fertilization  usually  takes 
place ;  a  spermatium  attaches 
itself  to  the  trichogyne  (Fig. 
28,  A,  s) ;  at  the  point  of 
contact  the  two  walls  become 
perforated,  and  the  contents 
of  the  spermatium  thus  enter 
the  trichogyne,  and  so  reach 
the  bulbous  base  of  the  oogo- 
nium. The  above  account  rep- 
resents the  very  simplest  con- 
ditions of  the  process  of  fer- 
tilization in  this  group,  and 
gives  no  idea  of  the  great  and 
puzzling  complexity  exhibited 
by  the  majority  of  forms. 

After  fertilization  the  trich- 
ogyne wilts,  and  the  bulbous 
base  in  one  way  or  another 
develops  a  conspicuous  struc- 
ture called  the  cystocarp  (Figs.  28,  29),  which  is  a  case  con- 
taining asexual  spores ;  in  other  words,  a  spore  case,  or  kind 
of  sporangium.     In  the  life  history  of  a  red  alga,  there- 


Fig-  29.  A  branch  of  Polysiphonia, 
one  of  the  red  algae,  showing  the 
rows  of  cells  composing  the  body 
(.4),  small  branches  or  hairs  (B), 
and  a  cystocarp  (C)  with  escaping 
spores  (Z>)  which  have  no  cilia  (car- 
pospores).— Caldwell. 


THE   GREAT   GROUPS   OF   ALG.E 


45 


fore,  two  sorts  of  asexual  spores  are  produced :  (1)  the 
tetraspores,  developed  in  ordinary  sporangia ;  and  (2)  the 
carpospores,  developed  in  the  cystocarp,  which  has  been 
produced  as  the  result  of  fertilization. 

OTHER   CHLOROPHYLL-CONTAINING   THALLOPHYTES 

34.  Diatoms. — These  are  peculiar  one-celled  forms,  which 
occur  in  very  great  abundance  in  fresh  and  salt  waters. 


Fig.  30.  A  group  of  Diatoms  :  c  and  d.  top  and  side  views  of  the  same  form;  e,  colony 
of  stalked  forms  attached  to  an  alga;/  and  g,  top  and  side  views  of  the  form  shown 
at  e;  A,  a  colony;  i,  a  colony,  the  top  and  side  view  shown  at  A;.— After  Kerner. 


They  are  either  free-swimming  or  attached  by  gelatinous 
stalks;  solitary,  or  connected  in  bands  or  chains,  or  im- 
bedded in  gelatinous  tubes  or  masses.  In  form  they  are 
rod-shaped,  boat-shaped,  elliptical,  wedge-shaped,  straight 
or  curved  (Fig.  30). 
22 


46 


PLANT   STRUCTURES 


The  chief  peculiarity  is  that  the  wall  is  composed  of  two 
valves,  one  of  which  fits  into  the  other  like  the  two  parts  of 
a  pill  box.  This  wall  is  so  impregnated  with  silica  that  it 
is  practically  indestructible,  and  siliceous  skeletons  of  dia- 
toms are  preserved  abundantly  in  certain  rock  deposits. 
They  multiply  by  cell  division  in  a  peculiar  way,  and  some 
of  them  have  been  observed  to  con- 
jugate. 

They  occur  in  such  numbers  in  the 
ocean  that  they  form  a  large  part  of 
the  free-swimming  forms  on  the  sur- 
face of  the  sea,  and  doubtless  showers 
of  the  siliceous  skeletons  are  constant- 
ly falling  on  the  sea  bottom.  There 
are  certain  deposits  known  as  "si- 
liceous earths,"  which  are  simply 
masses  of  fossil  diatoms. 

Diatoms  have  been  variously  placed 
in  schemes  of  classification.  Some 
have  put  them  among  the  Brown 
Algae  because  they  contain  a  brown 
coloring  matter;  others  have  placed 
them  in  the  Conjugate  forms  among 
the  Green  Algae  on  account  of  the 
occasional  conjugation  that  has  been 
observed.  They  are  so  different  from 
other  forms,  however,  that  it  seems 
best  to  keep  them  separate  from  all 
other  Algae. 

35.  Characeae. — These  are  common- 
ly called  "  stoneworts,"  and  are  often 
included  as  a  group  of  Green  Algae, 
as  they  seem  to  be  Thallophytes,  and 
have  no  other  coloring  matter  than 
chlorophyll.  However,  they  are  so  peculiar  that  they  are 
better  kept  by  themselves  among  the  Algae.    They  are  such 


Pig.  31.  A  common  Chara, 
showing  tip  of  main  axis. 
—After  Strasburger. 


THE  GREAT  GEODPS  OF  ALG^E  4f 

specialized  forms,  and  are  so  much  more  highly  organized 
than  all  other  Algge,  that  they  will  be  passed  over  here  with 
a  bare  mention.  They  grow  in  fresh  or  brackish  waters, 
fixed  to  the  bottom,  and  forming  great  masses.  The  cylin- 
drical stems  are  jointed,  the  joints  sending  out  circles  of 
branches,  which  repeat  the  jointed  and  branching  habit 
(Fig.  31). 

The  walls  become  incrusted  with  a  deposit  of  lime, 
which  makes  the  plants  harsh  and  brittle,  and  has  sug- 
gested the  name  "  stoneworts."  In  addition  to  the  highly 
organized  nutritive  body,  the  antheridia  and  oogonia  are 
peculiarly  complex,  being  entirely  unlike  the  simple  sex 
organs  of  the  other  Algae. 


CHAPTEE  V 

THALLOPHYTES :  FUNGI 

36.  General  characters. — In  general,  Fungi  include  Thai- 
lophytes  which  do  not  contain  chlorophyll.  From  this  fact 
it  follows  that  they  can  not  manufacture  food  entirely  out 
of  inorganic  material,  but  are  dependent  for  it  upon  other 
plants  or  animals.  This  food  is  obtained  in  two  general 
ways,  either  (1)  directly  from  the  living  bodies  of  plants  or 
animals,  or  (2)  from  dead  bodies  or  the  products  of  living 
bodies.  In  the  first  case,  in  which  living  bodies  are  at- 
tacked, the  attacking  fungus  is  called  a  parasite,  and  the 
plant  or  animal  attacked  is  called  the  host.  In  the  second 
case,  in  which  living  bodies  are  not  attacked,  the  fungus  is 
called  a  saprophyte.  Some  Fungi  can  live  only  as  parasites, 
or  as  saprophytes,  but  some  can  live  in  either  way. 

Fungi  form  a  very  large  assemblage  of  plants,  much 
more  numerous  than  the  Algae.  As  many  of  the  parasites 
attack  and  injure  useful  plants  and  animals,  producing 
many  of  the  so-called  "  diseases,"  they  are  forms  of  great 
interest.  Governments  and  Experiment  Stations  have  ex- 
pended a  great  deal  of  money  in  studying  the  injurious 
parasitic  Fungi,  and  in  trying  to  discover  some  method  of 
destroying  them  or  of  preventing  their  attacks.  Many  of 
the  parasitic  forms,  however,  are  harmless ;  while  many  of 
the  saprophytic  forms  are  decidedly  beneficial. 

It  is  generally  supposed  that  the  Fungi  are  derived  from 
the  Algse,  having  lost  their  chlorophyll  and  power  of  inde- 
pendent living.  Some  of  them  resemble  certain  Algae  so 
closely  that  the  connection  seems  very  plain;  but  others 
48 


THALLOPHYTES:  FUNGI 


49 


have  been  so  modified  by  their  parasitic  and  saprophytic 
habits  that  they  have  lost  all  likeness  to  the  Algae,  and 
their  connection  with  them  is  very  obscure. 

37.  The  plant  body. — Discarding  certain  problematical 
forms,  to  be  mentioned  later,  the  bodies  of  all  true  Fungi 
are  organized  upon  a  uniform  general  plan,  to  which  they 
can  all  be  referred  (Fig.  32).     A  set  of  colorless  branching 


Pig.  32.  A  diagrammatic  representation  of  Mucor,  showing  the  profusely  branching 
mycelium,  and  three  vertical  hyphre  (sporophores),  sporangia  forming  on  b  and  c. 
—After  Zopf. 


filaments,  either  isolated  or  interwoven,  forms  the  main 
working  body,  and  is  called  the  mycelium.  The  interweav- 
ing may  be  very  loose,  the  mycelium  looking  like  a  delicate 
cobweb ;  or  it  may  be  close  and  compact,  forming  a  felt-like 
mass,  as  may  often  be  seen  in  connection  with  preserved 
fruits.  The  individual  threads  are  called  hyplim  (singular, 
liypha)  or  hyphal  threads.  The  mycelium  is  in  contact  with 
its  source  of  food  supply,  which  is  called  the  substratum. 


50  PLANT  STRUCTURES 

From  the  hyplial  threads  composing  the  mycelium  verti- 
cal ascending  branches  arise,  which  are  set  apart  to  produce 
the  asexual  spores,  which  are  scattered  and  produce  new 
mycelia.  These  branches  are  called  ascending  liyphce  or 
sporophores,  meaning  "  spore  bearers." 

Sometimes,  especially  in  the  case  of  parasites,  special 
descending  branches  are  formed,  which  penetrate  the  sub- 
stratum or  host  and  absorb  the  food  material.  These  spe- 
cial absorbing  branches  are  called  haustoria,  meaning  "  ab- 
sorbers." 

Such  a  mycelial  body,  with  its  sporophores,  and  perhaps 
haustoria,  lies  either  upon  or  within  a  dead  substratum  in 
the  case  of  saprophytes,  or  upon  or  within  a  living  plant  or 
animal  in  the  case  of  parasites. 

38.  The  subdivisions. — The  classification  of  Fungi  is  in 
confusion  on  account  of  lack  of  knowledge.  They  are  so 
much  modified  by  their  peculiar  life  habits  that  they  have 
lost  or  disguised  the  structures  which  prove  most  helpful  in 
classification  among  the  Alga?.  Four  groups  will  be  pre- 
sented, often  made  to  include  all  the  Fungi,  but  doubtless 
they  are  insufficient  and  more  or  less  unnatural. 

The  constant  termination  of  the  group  names  is  mycetes, 
a  Greek  word  meaning  "  fungi."  The  prefix  in  each  case  is 
intended  to  indicate  some  important  character  of  the  group. 
The  names  of  the  four  groups  to  be  presented  are  as  follows : 
(1)  Phycomycetes  ("  Alga-Fungi "),  referring  to  the  fact 
that  the  forms  plainly  resemble  the  Algae  ;  (2)  Ascomycetes 
("  Ascus-Fungi  ") ;  (3)  ^Ecidiomycetes  ("iEcidium-Fungi  ") ; 
(4)  Basidiomycetes  ("  Basidium-Fungi ").  Just  what  the 
prefixes  ascus,  cecidium,  and  basidium  mean  will  be  ex- 
plained in  connection  with  the  groups.  The  last  three 
groups  are  often  associated  together  under  the  name  My- 
comycetes,  meaning  "  Fungus-Fungi,"  to  distinguish  them 
from  the  Phycomycetes,  or  "  Alga-Fungi,"  referring  to  the 
fact  that  they  do  not  resemble  the  Algge,  and  are  only  like 
themselves. 


THALLOPHYTES:  FUNGI 


51 


One  of  the  ordinary  life  processes  which  seems  to  be 
seriously  interfered  with  by  the  saprophytic  and  parasitic 
habit  is  the  sexual  process.  At  least,  while  sex  organs 
and  sexual  spores  are  about  as  evident  in  Phycomycetes 
as  in  Algas,  they  are  either  obscure  or  wanting  in  the 
Mycomycete  groups. 

1.  Phycomycetes  {Alga-Fungi) 

39.  Saprolegnia. — This  is  a  group  of  "water-moulds," 
with  aquatic  habit  like  the  Algae.  They  live  upon  the  dead 
bodies  of  water  plants  and  animals  (Fig.  33),  and  some- 
times attack  living  fish,  one  kind  being  very  destructive 
to  young  fish  in  hatcheries.  The  hyphse  composing  the 
mycelium  are  ccenocytes,  as  in  the  Siphon  forms. 

Sporangia  are  organized  at  the  ends  of  branches  by 
forming  a  partition  wall  separating  the  cavity  of  the  tip 
from  the  general  cavity  (Fig.  33,  B).  The  tip  becomes 
more  or  less  swollen,  and  within  it  are  formed  numerous 
biciliate  zoospores,  which  are  discharged  into  the  water 
(Fig.  33,  0),  swim  about  for  a  short  time,  and  rapidly  form 
new  mycelia.  The  process  is  very  suggestive  of  Cladophora 
and  Vaucheria.  Oogonia  and  antheridia  are  also  formed 
at  the  ends  of  the  branches  (Fig.  33,  F),  much  as  in  Vau- 
cheria. The  oogonia  are  spherical,  and  form  one  and  some- 
times many  eggs  (Fig.  33,  D,  E).  The  antheridia  are 
formed  on  branches  near  the  oogonia.  An  antheridium 
conies  in  contact  with  an  oogonium,  and  sends  out  a  deli- 
cate tube  which  pierces  the  oogonium  wall  (Fig.  33,  F). 
Through  this  tube  the  contents  of  the  antheridium  pass, 
fuse  with  the  egg,  and  a  heavy-walled  oospore  or  resting 
spore  is  the  result. 

It  is  an  interesting  fact  that  sometimes  the  contents  of 
an  antheridium  do  not  enter  an  oogonium,  or  antheridia 
may  not  even  be  formed,  and  still  the  egg,  without  fertiliza- 
tion, forms  an  oospore, which  can  germinate.     This  peculiar 


52 


PLANT   STKUCTUKES 


habit  is  called  parthenogenesis,  which  means  reproduction 
by  an  egg  without  fertilization. 


Fig.  33.  A  common  water  mould  (Saprolegnia) :  A,  a  fly  from  which  mycelial  fila- 
ments of  the  parasite  are  growing;  B,  tip  of  a  branch  organized  as  a  sporangium; 
C,  sporangium  discharging  biciliate  zoospores;  F,  oogonium  with  antheridium  in 
contact,  the  tube  having  penetrated  to  the  egg;  D  and  E,  oogonia  with  several 
eggs.— A- C after  Thuret.  D-F after  DeBart. 

40.  Mucor. — One  of  the  most  common  of  the  Mucors,  or 
"  black  moulds,"  forms  white  furry  growths  on  damp  bread, 
preserved  fruits,  manure  heaps,  etc.  It  is  therefore  a 
saprophyte,  the  ccenocytic  mycelium  branching  extensively 
through  the  substratum  (Fig.  34). 


THALLOPHYTES  :  FUNGI 


53 


Erect  sporophores  arise  from  it  in  abundance,  and  at 
the  top  of  each  sporophore  a  globular  sporangium  is  formed, 
within  which  are  numerous  small  asexual  spores  (Figs.  35, 


Fig.  34.    Diagram  showing  mycelium  and  sporophores  of  a  common  Mucor. — 
Caldwell. 


36).  The  sporangium  wall  bursts  (Fig.  37),  the  light  spores 
are  scattered  by  the  wind,  and,  falling  upon  a  suitable  sub- 
stratum, germinate  and 
form  new  mycelia.  It  is 
evident  that  these  asex- 
ual spores  are  not  zoo- 
spores, for  there  is  no 
water  medium  and  swim- 
ming is  impossible.  This 
method  of  transfer  being 
impossible,  the  spores  are 
scattered  by  currents  of 
air,  and  must  be  corre- 
spondingly light  and  pow- 
dery. They  are  usually 
spoken  of  simply  as 
"  spores,"  without  any 
prefix. 


Fig.  35.  Forming  sporangia  of  Mucor,  show- 
ing the  swollen  tip  of  the  sporophore  (A), 
and  a  later  stage  (B),  in  which  a  wall  is 
formed  separating  the  sporangium  from 
the  rest  of  the  body.— Caldwell. 


54 


PLANT   STRUCTURES 


While  the  ordinary  method  of  reproduction  through  the 
growing  season  is  by  means  of  these  rapidly  germinating 
spores,  in  certain  conditions  a  sexual  process  is  observed, 
by  which  a  heavy-walled  sexual  spore  is  formed  as  a  resting 
spore,  able  to  outlive  unfavorable  conditions.  Branches 
arise  from  the  hyphae  of  the  mycelium  just  as  in  the  forma- 


Fig.  36.  Mature  sporangium  of  Mucor,  showing 
the  wall  (A),  the  numerous  spores  (C),  and 
the  columella  (B)— that  is,  the  partition  wall 
pushed  up  into  the  cavity  of  the  sporangium. 
—Caldwell. 


Fig.  37.  Bursted  sporangium  of 
Mucor,  the  ruptured  wall  not 
being  shown,  and  the  loose 
spores  adhering  to  the  colu- 
mella.— Caldwell. 


tion  of  sporophores  (Fig.  38).  Two  contiguous  branches 
come  in  contact  by  their  tips  (Fig.  38,  A),  the  tips  are  cut 
off  from  the  main  coenocytic  body  by  partition  walls  (Fig. 
38,  B),  the  walls  in  contact  disorganize,  the  contents  of 
the  two  tip  cells  fuse,  and  a  heavy-walled  sexual  spore  is 
the  result  (Fig.  38,  C).  It  is  evident  that  the  process  is 
conjugation,  suggesting  the  Conjugate  forms  among  the 


TIIALLOPHYTES:   FUNGI 


55 


Algae  ;  that  the  sexual  spore  is  a  zygote  ;  and  that  the  two 
pairing  tip  cells  cut  off  from  the  main  body  by  partition 
walls  are  gametangia.     Mucor,  therefore,  is  isogamous. 


Fig.  38.  Sexual  reproduction  of  Mucor,  showing  tips  of  sex  branches  meeting  (A), 
the  two  gametangia  cut  off  by  partition  walls  (B),  and  the  heavy-walled  zygote 
(  C).— Caldwell. 


41.  Peronospora. — These  are  the  "  downy  mildews,"  very 
common  parasites  on  seed  plants  as  hosts,  one  of  the  most 
common  kind  attacking  grape  leaves.  The  mycelium  is  cceno- 
cytic  and  entirely  internal,  ramifying  among  the  tissues 
within  the  leaf,  and  piercing  the  living  cells  with  haustoria 
which  rapidly  absorb  their  contents  (Fig.  39).  The  pres- 
ence of  the  parasite   is   made   known  by  discolored  and 


56 


PLANT  STRUCTURES 


finally  deadened  spots  on  the  leaves,  where  the  tissues  have 
been  killed. 

From  this  internal  mycelium  numerous  sporophores 
arise,  coming  to  the  surface  of  the  host  and  securing  the 
scattering  of  their 
spores,  which  fall 
upon  other  leaves 
and  germinate,  the 
new  mycelia  pene- 
trating among  the 
tissues  and  begin- 
ning their  ravages. 
The  sporophores,  af- 
ter rising  above  the 
surface   of  the  leaf, 

branch  freely ;  and  many  of  them  rising  near  together, 
they  form  little  velvety  patches  on  the  surface,  suggesting 
the  name  "  downy  mildew." 

b  c 


Fig.  39.  A  branch  of  Peronospora  in  contact  with 
two  cells  of  a  host  plant,  and  sending  into  them 
its  large  haustoria.—  After  DeBary. 


Fig.  40.  Peronospora,  one  of  the  Phycomycetes,  showing  at  a  an  oogonium  (o)  con- 
taining an  egg,  and  an  antheridium  (»)  in  contact;  at  b  the  antheridial  tube  pene- 
trating the  oogonium  and  discharging  the  contents  of  the  antheridium  into  the 
egg;  at  c  the  oogonium  containing  the  oospore  or  resting  spore. — After  DeBary. 


In  certain  conditions  special  branches  arise  from  the 
mycelium,  which  organize  antheridia  and  oogonia,  and 
remain  within  the  host  (Fig.  40).  The  oogonium  is  of  the 
usual  spherical  form,  organizing  a  single  egg.     The  an- 


THALLOFHYTES :  FUNGI 


57 


theridium  comes  in  contact  with  the  oogonium,  puts  out  a 
tube  which  pierces  the  oogonium  wall  and  enters  the  egg, 
into  which  the  contents  of  the  antheridium  are  discharged, 
and  fertilization  is  effected.  The  result  is  a  heavy-walled 
oospore.  As  the  oospores  are  not  for  immediate  germina- 
tion, they  are  not  brought  to  the  surface  of  the  host  and 
scattered,  as  are  the  asexual  spores.  When  they  are  ready 
to  germinate,  the  leaves  bearing  them  have  perished  and 
the  oospores  are  liberated. 

42.  Conclusions. — The  coenocytic  bodies  of  the  whole  group 
are  very  suggestive  of  the  Siphon  forms  among  Green  Alga?, 
as  is  also  the  method  of  forming  oogonia  and  antheridia. 

The  water-moulds,  Saprolegnia  and  its  allies,  have  re- 
tained the  aquatic  habit  of  the  Algae,  and  their  asexual 
spores  are  zoospores.  Such  forms  as  Mucor  and  Perono- 
spora,  however,  have  adapted  themselves  to  terrestrial  con- 
ditions, zoospores  are  abandoned,  and  light  spores  are  de- 
veloped which  can  be  carried  about  by  currents  of  air. 

In  most  of  them  motile  gametes  are  abandoned.  Even 
in  the  heterogamous  forms  sperms  are  not  organized  within 
the  antheridium,  but  the  contents  of  the  antheridium  are 
discharged  through  a  tube  developed  by  the  wall  and  pene- 
trating the  oogonium.  It  should  be  said,  however,  that  a 
few  forms  in  this  group  develop  sperms,  which  make  them 
all  the  more  alga-like. 

They  are  both  isogamous  and  heterogamous,  both  zygotes 
and  oospores  being  resting  spores.  Taking  the  characters 
all  together,  it  seems  reasonably  clear  that  the  Phycomycetes 
are  an  assemblage  of  forms  derived  from  Green  Algae  (Chlo- 
rophyceae)  of  various  kinds. 

2.  Ascomycetes  (Ascus-  or  Sac-Fungi) 

43.  Mildews. — These  are  very  common  parasites,  growing 
especially  upon  leaves  of  seed  plants,  the  mycelium  spread- 
ing over  the  surface  like  a  cobweb.     A  very  common  mil- 


58 


PLANT   STRUCTURES 


dew,  Microsphcera,  grows  on  lilac  leaves,  which  nearly 
always  show  the  whitish  covering  after  maturity  (Fig.  41). 
The  branching  hyphas  show  numerous  partition  walls,  and 
are  not  ccenocytic  as  in  the  Phycomycetes.  Small  disk-like 
haustoria  penetrate  into  the  superficial  cells  of  the  host, 
anchoring  the  mycelium  and  absorbing  the  cell  contents. 

Sporophores  arise,  which  form  asexual  spores  in  a  pe- 
culiar way.  The  end  of  the  sporophore  rounds  off,  almost 
separating  itself  from  the  part  below,  and  becomes  a  spore 
or  spore-like  body.      Below  this  another  organizes  in  the 

same  way,  then  another,  until 
a  chain  of  spores  is  developed, 
easily  broken  apart  and  scat- 
tered by  the  wind.  Falling 
upon  other  suitable  leaves, 
they  germinate  and  form  new 
mycelia,  enabling  the  fungus 
to  spread  rapidly.  This  meth- 
od of  cutting  a  branch  into 
sections  to  form  spores  is 
called  abstriction,  and  the 
spores  formed  in  this  way 
are  called  conidia,  or  conidi- 
ospores  (Fig.  43,  B). 

At  certain  times  the  myce- 
lium develops  special  branches 
which  develop  sex  organs,  but 
they  are  seldom  seen  and  may 
not  always  occur.     An  oogo- 
nium and  an  antheridium,  of 
the  usual  forms,  but  probably 
without   organizing   gametes, 
come  into  contact,  and  as  a 
result  an  elaborate  structure  is  developed — the  ascocarp, 
sometimes  called  the  "spore  fruit."     These  ascocarps  ap- 
pear on  the  lilac  leaves  as  minute  dark  dots,  each  one  being 


Fig.  41.  Lilac  leaf  covered  with  mil- 
dew (Mic?-0fi])hcera),  the  shaded  re- 
gions representing  the  mycelium, 
and  the  black  dots  the  ascocarps.— 
S.  M.  Coulter. 


THALLOPHYTES:  FUNGI 


59 


£&§? 


a  little  sphere,  which  suggested  the  name  Microsphcera 
(Fig.  41).  The  heavy  wall  of  the  ascocarp  bears  beauti- 
ful branching  hair-like  appendages  (Fig.  42). 

Bursting  the  wall  of  this  spore  fruit  several  very  delicate, 
bladder-like  sacs  are  extruded,  and  through  the  transparent 
wall  of  each  sac  there  may  be 
seen  several  spores  (Fig.  42). 
The  ascocarp,  therefore,  is 
a  spore  case,  just  as  is  the 
cystocarp  of  the  Red  Algae 
(§  33).  The  delicate  sacs 
within  are  the  asci,  a  word 
meaning  "sacs,"  and  each 
ascus  is  evidently  a  mother 
cell  within  which  asexual 
spores  are  formed.  These 
spores  are  distinguished 
from  other  asexual  spores 
by  the  name  ascospore. 

It  is  these  peculiar  moth- 
er cells,  or  asci,  which  give 
name  to  the  group,  and  an 

Ascomycete,  Ascus-fungus,  or  Sac-fungus,  is  one  which  pro- 
duces spores  in  asci ;  and  an  ascocarp  is  a  spore  case  which 
contains  asci. 

In  the  mildews,  therefore,  there  are  two  kinds  of  asexual 
spores  :  (1)  conidia,  formed  from  a  hyphal  branch  by  abstric- 
tion,  by  which  the  mycelium  may  spread  rapidly;  and  (2) 
ascospores,  formed  in  a  mother  cell  and  protected  by  a  heavy 
case,  so  that  they  may  bridge  over  unfavorable  conditions, 
and  may  germinate  when  liberated  and  form  new  mycelia. 
The  resting  stage  is  not  a  zygote  or  an  oospore,  as  in  the 
Alga?  and  Phycomycetes,  no  sexual  spore  probably  being 
formed,  but  a  heavy-walled  ascocarp. 

44.  Other  forms. — The  mildews  have  been  selected  as  a 
simple  illustration  of  Ascomycetes,  but  the  group  is  a  very 


Fig.  42.  Ascocarp  of  the  lilac  mildew, 
showing  branching  appendages  and 
two  asci  protruding  from  the  rup- 
tured wall  and  containing  ascospores. 
— S.  M.  Coulter. 


60 


PLANT  STRUCTURES 


large  one,  and  contains  a  great  variety  of  forms.  All  of 
them,  however,  produce  spores  in  asci,  but  the  asci  are  not 
always  inclosed  by  an  ascocarp.  Here  belong  the  common 
blue  mould  {Penicillium),  found  on  bread,  fruit,  etc.,  in 
which  stage  the  branching  chains  of  conidia  are  very  con- 
spicuous (Fig.  43) ;  the  truffle-fungi,  upon  whose  subter- 


Fig.  43.  Penicillium,  a  common  mould:  A,  mycelium  with  numerous  branching 
sporophores  bearing  conidia;  B,  apex  of  a  sporophore  enlarged,  showing  branch- 
ing and  chains  of  conidia. — After  Brefeld. 


ranean  mycelia  ascocarps  develop  which  are  known  as 
"  truffles  "  ;  the  black  fungi,  which  form  the  diseases  known 
as  "  black  knot  "  of  the  plum  and  cherry,  the  "  ergot  "  of 
rye  (Fig.  44),  and  many  black  wart-like  growths  upon  the 
bark  of  trees  ;  other  forms  causing  "  witches'-brooms  "  (ab- 
normal growths  on  various  trees),  "  peach  curl,"  etc.,  the 
cup-fungi  (Figs.  45,  46),  and  the  edible  morels  (Fig.  47). 


THALLOPHYTES:  FUNGI 


61 


Vie.  44.  Head  of  rye  attacked  by  "  er- 
got" (a),  peculiar  grain-like  masses 
replacing  the  grains  of  rye  ;  also  a 
mass  of  "ergot"  germinating  to 
form  spores  (6).— After  Tttlasne. 


Fig.  46.  A  cup-fungus  (Pitya)  grow- 
ing on  a  spruce  (Picea).  —  After 
Rehm. 


In  some  of  these  forms  the  ascocarp  is  completely  closed, 
as  in  the  lilac  mildew;  in  others  it  is  flask-shaped;    in 
others,  as  in  the  cup-fungi,  it  is  like  a  cup  or  disk  ;  but  in 
all  the  spores  are  inclosed  by  a  delicate  sac,  the  ascus. 
23 


62 


i'LANT   STRUCTURES 


Here  must  probably  be  included  the  yeast-fungi  (Fig. 
48),  so  commonly  used   to  excite  alcoholic  fermentation. 


Fig.  47.  The  common  edible  morel  (Morrhella 
esculenta).  The  structure  shown  and  used 
represents  the  ascocarp,  the  depressions  of 
whose  surface  are  lined  with  asci  contain- 
ing ascospores. — After  Gibson. 


Fig.  48.  Yeast  cells,  reprodu- 
cing by  budding,  and  form- 
ing chains. — Caldwell. 


The  "  yeast  cells  "  seem  to  be  conidia  having  a  peculiar  bud- 
ding method  of  multiplication,  and  the  remarkable  power 
of  exciting  alcoholic  fermentation  in  sugary  solutions. 

3.  JEcidiomycetes  {JEcidium-Fungi) 

45.  General  characters. — This  is  a  large  group  of  very 
destructive  parasites  known  as  "  rusts  "  and  "  smuts."  The 
rusts  attack  particularly  the  leaves  of  higher  plants,  pro- 
ducing rusty  spots,  the  wheat  rust  probably  being  the  best 
known.  The  smuts  especially  attack  the  grasses,  and  are 
very  injurious  to  cereals,  producing  in  the  heads  of  oats, 
barley,  wheat,  corn,  etc.,  the  disease  called  smut. 


THALLOPIIYTES  ?  FUNGI  (53 

In  some  forms  an  obscure  sexual  process  has  been  de- 
scribed,  but  it  is  beyond  the  reach  of  ordinary  observation. 
The  iEcidiomycetes  do  not  form  an  independent  and  nat- 
ural group,  but  are  now  generally  placed  under  the  Basi- 
diomycetes,  but  they  are  so  unlike  the  ordinary  forms  of 
that  group  that  they  are  here  kept  distinct. 

Most  of  the  forms  are  very  polymorphic — that  is,  a  plant 
assumes  several  dissimilar  appearances  in  the  course  of  its 
life  history.  These  phases  are  often  so  dissimilar  that  they 
have  been  described  as  different  plants.  This  polymorphism 
is  often  further  complicated  by  the  appearance  of  different 
phases  upon  entirely  different  hosts.  For  example,  the 
wheat-rust  fungus  in  one  stage  lives  on  wheat,  and  in  an- 
other »on  barberry. 

46.  Wheat  rust. — This  is  one  of  the  few  rusts  whose  life 
histories  have  been  traced,  and  it  may  be  taken  as  an  illus- 
tration of  the  group. 

The  mycelium  of  the  fungus  is  found  ramifying  among 
the  leaf  and  stem  tissues  of  the  wheat.  While  the  wheat  is 
growing  this  mycelium  sends  to  the  surface  numerous  spo- 


cg^^lfllHg^?D  lIliJL 


Fig.  49.    Wheat  rust,  showing  sporophores  breaking  through  the  tissues  of  the  host 
and  bearing  summer  spores  (uredospores).—  After  H.  Marshall  Ward. 

rophores,  each  bearing  at  its  apex  a  reddish  spore  (Fig.  49). 
As  the  spores  occur  in  great  numbers  they  form  the  rusty- 
looking  lines  and  spots  which  give  name  to  the  disease. 
The  spores  are  scattered  by  currents  of  air,  and  falling  upon 
other  plants,  germinate  very  promptly,  thus  spreading  the 


64 


PLANT  STKUCTURES 


disease  with  great  rapidity  (Fig.  50).  Once  it  was  thought 
that  this  completed  the  life  cycle,  and  the  fungus  received 
the  name  Uredo.     When  it  was  known  that  this  is  but  one 


Fig.  50 


-Wheat  rust,  showing  a  young  hypha  forcing  its  way  from  the  surface  of  a 
leaf  down  among  the  nutritive  cells.— After  H.  Marshall  Ward. 


stage  in  a  polymorphic  life  history  it  was  called  the  Uredo- 
stage,  and  the  spores  uredospores,  sometimes  "summer 
spores." 


Fig.  51.    Wheat  rust,  showing  the  winter  spores  (teleutospores). — After 
H.  Marshall  Ward. 

Toward  the  end  of  the  summer  the  same  mycelium 
develops  sporophores  which  bear  an  entirely  different  kind 
of  spore  (Fig.  51).     It  is  two-celled,  with  a  very  heavy  black 


THALLOPHYTES :  FUNGI 


65 


wall,  and  forms  what  is  called  the  "  black  rust,"  which  ap- 
pears late  in  the  summer  on  wheat  stubble.  These  spores 
are  the  resting  spores,  which  last  through  the  winter  and 
germinate  in  the  following  spring.  They  are  called  teleuto- 
spore.%  meaning  the  "  last  spores  "  of  the  growing  season. 
They  are  also  called  "  winter  spores,"  to  distinguish  them 
from  the  uredospores  or  "  summer  spores."  At  first  this 
teleutospore-bearing  mycelium  was  not  recognized  to  be 
identical  with  the  uredospore-bearing  mycelium,  and  it  was 
called  Puccinia.  This  name  is  now 
retained  for  the  whole  polymorphous 
plant,  and  wheat  rust  is  Puccinia 
graminis.  This  mycelium  on  the 
wheat,  with  its  summer  spores  and 
winter  spores,  is  but  one  stage  in 
the  life  history  of  wheat  rust. 

In  the  spring  the  teleutospore 
germinates,  each  cell  developing  a 
small  few-celled  filament  (Fig.  52). 
From  each  cell  of  the  filament  a 
little  branch  arises  which  develops 
at  its  tip  a  small  spore,  called  a  spo- 
ridium,  which  means  "  spore-like." 
This  little  filament,  which  is  not  a 
parasite,  and  which  bears  sporidia, 
is  a  second  phase  of  the  wheat  rust, 
really  the  first  phase  of  the  growing 
season. 

The  sporidia  are  scattered,  fall 
upon  barberry  leaves,  germinate,  and 
develop  a  mycelium  which  spreads 
through  the  leaf.  This  mycelium  produces  sporophores 
which  emerge  on  the  under  surface  of  the  leaf  in  the 
form  of  chains  of  reddish-yellow  conidia  (Fig.  53).  These 
chains  of  conidia  are  closely  packed  in  cup-like  receptacles, 
and  these  reddish-yellow  cup-like  masses  are  often  called 


Fig.  52.  Wheat  rust,  show- 
ing a  teleutospore  germina- 
ting and  forming  a  short  fil- 
ament, from  four  of  whose 
cells  a  spore  branch  arises, 
the  lowest  one  bearing  at 
its  tip  a  sporidium. — After 
H.  Marshall  Ward. 


66 


PLANT   STRUCTURES 


"cluster-cups."     This  mycelium  on  the  barberry,  bearing 
cluster-cups,  was  thought  to  be  a  distinct  plant,  and  was 

called  JEcidium.  The 
name  now  is  applied  to 
the  cluster-cups,  which 
are  called  cecidia,  and 
the  conidia-like  spores 
which  they  produce  are 
known  as  cecidiospores. 

It  is  the  ascidia  which 
give  name  to  the  group, 
and  ^cidiomycetes  are 
those  Fungi  in  whose 
life  history  secidia  or 
cluster-cups  appear. 

The  secidiospores  are 
scattered  by  the  wind, 
fall  upon  the  spring 
wheat,  germinate,  and 
develop  again  the  myce- 
lium which  produces  the 
rust  on  the  wheat,  and 
so  the  life  cycle  is  com- 
pleted. There  are  thus 
at  least  three  distinct 
stages  in  the  life  history 
of  wheat  rust.  Begin- 
ning with  the  growing 
season  they  are  as  fol- 
lows :  (1)  The  phase  bear- 
ing the  sporidia,  which 
is  not  parasitic ;  (2)  the 
aecidium  phase,  parasitic 
on  the  barberry;  (3)  the  uredo-teleutospore  phase,  para- 
sitic on  the  wheat. 

In  this  life  cycle  at  least  four  kinds  of  asexual  spores 


THALLOPHYTES:  FUNGI 


67 


appear :  (1)  sporidia,  which  develop  the  stage  on  the  barber- 
ry ;  (2)  cecidiospores,  which  develop  the  stage  on  the  wheat ; 
(3)  uredospores,  which  repeat  the  mycelium  on  the  wheat ;  (4) 
teleutospores,which  last  through  the  winter,  and  in  the  spring 
produce  the  stage  bearing  sporidia.  It  should  be  said  that 
there  are  other  structures  of  this  plant  produced  on  the  bar- 
berry (Fig.  53),  but  they  are  too  uncertain  to  be  included  here. 
The  barberry  is  not  absolutely  necessary  to  this  life  cycle. 
In  many  cases  there  is  no  available  barberry  to  act  as  host, 
and  the  sporidia  germinate  directly  upon  the  young  wheat, 
forming  the  rust-producing  mycelium,  and  the  cluster-cup 
stage  is  omitted. 


Fig.  54.    Two  species  of  "cedar  apple"  (Gymnosporangium),  both  on  the  common 
juniper  (Juniperus  Virginiana).—A  after  Farlow,  B  after  Engleb  and  Prantl. 


47.  Other  rusts.— Many  rusts  have  life  histories  similar 
to  that  of  the  wheat  rust,  in  others  one  or  more  of  the 
stages  are  omitted.     In  very  few  have  the  stages  been  con- 


68 


PLANT   STBUCTUKES 


nected  together,  so  that  a  mycelium  bearing  uredospores  is 
called  a  Uredo,  one  bearing  teleutospores  a  Puccinia,  and 
one  bearing  aecidia  an  jEcidium  ;  but  what  forms  of  Uredo, 
Puccinia,  and  ^Ecidium  belong  together  in  the  same  life 
cycle  is  very  difficult  to  discover. 

Another  life  cycle  which  has  been  discovered  is  in  con- 
nection with  the  "  cedar  apples "  which  appear  on  red 
cedar  (Fig.  54).  In  the  spring  these  diseased  growths  be- 
come conspicuous,  especially  after  a  rain,  when  the  jelly- 
like masses  containing  the  orange-colored  spores  swell. 
This  corresponds  to  the  phase  which  produces  rust  in 
wheat.  On  the  leaves  of  apple  trees,  wild  crab,  hawthorn, 
etc.,  the  ascidium  stage  of  the  same  parasite  develops. 

4.  Basidiomycetes  (Basidium-Fungi). 
48.  General  characters. — This  group  includes  the  mush- 
rooms, toadstools,  and  puffballs.     They  are  not  destructive 

parasites,  as  are  many 
forms  in  the  preceding 
groups,  but  mostly  harm 
less  and  often  useful  sap- 
rophytes. They  must 
also  be  regarded  as  the 
most  highly  organized  of 
the  Fungi.  The  popular 
distinction  between  toad- 
stools and  mushrooms  is 
not  borne  out  by  botan- 
ical characters,  toadstool 
and  mushroom  being  the 
same  thing  botanically, 
and  forming  one  group, 
puffballs  forming  an- 
other. 

Fig.  55.    The  common  edible  mushroom,  As  in  ^cidiomycetes, 

Agaricus  campestris.— After  Gibson.  an  obscure  Sexual  process 


THALLOPHYTES:  FUNGI 


69 


is  reported.  The  life  history  seems  simple,  but  this  appar- 
ent simplicity  may  represent  a  very  complicated  history. 
The  structure  of  the  common  mushroom  (Agaricus)  will 
serve  as  an  illustration  of  the  group  (Fig.  55). 

49.  A  common 
mushroom.  —  The 
mycelium,  of  white 
branching  threads, 
spreads  extensively 
through  the  decay- 
ing substratum, 
and  in  cultivated 
forms  is  spoken  of 
as  the  "  spawn." 
Upon  this  myce- 
lium little  knob- 
like protuberances 
begin  to  arise,  grow- 
ing  larger  and 
larger,  until  they 
are  organized  into 
the  so-called 
"mushrooms." 
The  real  body  of 
the  plant  is  the 
white  thread  -  like 
mycelium,  while 
the  "  mushroom  " 
part  seems  to  rep- 
resent a  great  num- 
ber of  sporophores 
organized  together 
to  form  a  single 
complex  spore- 
bearing  structure. 

The  mushroom 


Fig.  50.  A  common  Agaricus  :  A,  section  throngh  one 
side  of  pileus,  showing  sections  of  the  pendent  gills; 
B.  section  of  a  gill  more  enlarged,  showing  the  cen- 
tral tissue,  and  the  broad  border  formed  by  the  ba- 
sidia :  C,  still  more  enlarged  section  of  one  side  of 
a  gill,  showing  the  club-shaped  basidia  standing  at 
right  angles  to  the  surface,  and  sending  out  a  pair 
of  small  branches,  each  of  which  bears  a  single  ba- 
sidiospore.— After  Sachs. 


*„• 


-    I 


.     -JJ 


THALLOPHYTES:   FUNGI 


VI 


has  a  stalk-like  portion,  the  stipe,  at  the  base  of  which  the 
slender  mycelial  threads  look  like  white  rootlets ;  and  an 
expanded,  umbrella-like  top  called  the  pileus.  From  the 
under  surface  of  the  pileus  there  hang  thin  radiating  plates, 
or  gills  (Fig.  55).  Each  gill  is  a  mass  of  interwoven  fila- 
ments (hyphas),  whose  tips  turn  toward  the  surface  and 
form  a  compact  layer  of  end  cells  (Fig.  56).     These  end 


Fig.  60.    A  bracket  fungus  (Polyporus)  growing  on  the  trunk  of  a  red  oak.— 
Caldwell. 


cells,  forming  the  surface  of  the  gill,  are  club-shaped,  and 
are  called  basidia.  From  the  broad  end  of  each  basidium 
two  or  four  delicate  branches  arise,  each  bearing  a  minute 
spore,  very  much  as  the  sporidia  appear  in  the  wheat  rust. 


72 


PLANT   STRUCTURES 


These  spores,  called  basidiospores,  shower  down  from  the 
gills  when  ripe,  germinate,  and  produce  new  mycelia.  The 
peculiar  cell  called  the  basidium  gives  name  to  the  group 
Basidiomycetes. 

50.  Other  forms. — Mushrooms  display  a  great  variety  of 
form  and  coloration,  many  of  them  being  very  attractive 


.( 

h\ 

a     /  y/f 

IV 

1 s^Z^i^^ 

-  -  J^i^ 

j&JSf'"  \ 

ISlk  - 

/            H>B 

/■At  "*^B 

Pig.  61.    A  toadstool  of  the  bracket  form  which  has  grown  about  blades  of  grass 
without  interfering  with  their  activity.— Caldwell. 


(Figs.  57,  58,  59).  The  "  pore-fungi  "  have  pore-like  depres- 
sions for  their  spores,  instead  of  gills,  as  in  the  very  com- 
mon "bracket-fungus"  (Polyporus),  which  forms  hard 
shell-like  outgrowths  on  tree-trunks  and  stumps  (Figs.  60, 


Pig.  62.   The  common  edible  Boletus  (B.  edu-         Fig.  63.    Another  edible  Boletus  (B.  scro- 
lls), in  which  the  gills  are  replaced  by  bilaceus).— After  Gibson. 
pores.— After  Gibson. 


Fig.  64.    The  common  edible  "coral  fun-         Fig.  65.    Hydnum  repandum,  in  which  gills 
rob"  iClavaria).— After  Gibson.  are  replaced  by  spinous  processes;  edi- 

ble.—After  Gibson. 


74 


PLANT   STKUCTUKES 


61),  and  the  mushroom-like  Boleti  (Figs.  62,  63).  The 
"  ear-fungi "  form  gelatinous,  dark-brown,  shell-shaped 
masses,  and  the  "  coral  fungi "  resemble  branching  corals 
(Fig.   64).     The   Hydnum  forms  have   spinous  processes 

instead  of  gills  (Fig. 
65).  The  puffballs  or- 
ganize globular  bodies 
(Fig.  06),  within  which 
the  spores  develop,  and 
are  not  liberated  until 
ripe ;  and  with  them 
belong  also  the  "bird's 
nest  fungus,"  the  "earth 
star,"  the  ill-smelling 
"stink-horn,"  etc. 

OTHER  THALLOPHYTES 
WITHOUT  CHLOROPHYLL 

51 .   Slime  -  moulds.  — 

These  perplexing  forms, 
named  Myxomycetes,  do 
not  seem  to  be  related 
to  any  group  of  plants, 
and  it  is  a  question 
whether  they  are  to  be  regarded  as  plants  or  animals.  The 
working  body  is  a  mass  of  naked  protoplasm  called  a  plas- 
modium,  suggesting  the  term  "slime,"  and  slips  along  like 
a  gigantic  amoeba.  They  are  common  in  forests,  upon 
black  soil,  fallen  leaves,  and  decaying  logs,  the  slimy  yel- 
low or  orange  masses  ranging  from  the  size  of  a  pinhead 
to  as  large  as  a  man's  hand.  They  are  saprophytic,  and 
are  said  to  engulf  food  as  do  the  amcebas.  So  suggestive 
of  certain  low  animals  is  this  body  and  food  habit  that 
slime-moulds  have  also  been  called  Mycetozoa  or  "  fungus- 
animals." 


Fig.  66.    Puffballs,  in  which  the  basidia  and 
spores  are  inclosed  ;  edible. — After  Gibson. 


THALLOPIIYTES:  FUNGI 


75 


In  certain  conditions,  however,  these  slimy  bodies  come 
to  rest  and  organize  most  elaborate  and  often  very  beau- 
tiful sporangia,  full  of  spores  (Fig.  07).  These  varied 
and  easily  preserved   sporangia  are   used   to   classify  the 


Fig.  67.  Three  common  slime  moulds  (Myxomycetee)  on  decaying  wood:  to  the 
left  above,  groups  of  ihe  sessile  sporangia  of  Trichia  ;  to  the  right  above,  a  group 
of  the  stalked  sporangia  of  Stemonitis,  with  remnant  of  old  Plasmodium  at  base; 
below,  groups  of  sporangia  of  Hemiarcyria,  with  a  Plasmodium  mass  at  upper 
left  hand.— Goldbekqer. 


forms.  Slime-moulds,  or  "  slime-fungi,"  therefore,  seem 
to  have  animal-like  bodies  which  produce  plant-like  spo- 
rangia. 

52.  Bacteria. — These  are  the  "  Fission-Fungi,"  or  Schizo- 
mycetes,  and  are  popularly  known  as  "  bacteria,"  "  bacLli," 
"  microbes,"  "  germs,"  etc.  They  are  so  important  and  pe- 
culiar in  their  life  habits  that  their  study  has  developed  a 
special  branch  of  botany,  known  as  "  Bacteriology."  In 
many  ways  they  resemble  the  Cyanophyceae,  or  "Fission- 
Algae,"  so  closely  that  they  are  often  associated  with  them 
in  classification  (see  §  21). 


r  r 


Fig.  68.  A  group  of  Bacteria,  the  bodies  being  black,  and  bearing  motile  cilia  in 
various  ways.  A,  the  two  to  the  left  the  common  hay  Bacillus  (B.  subtilis),  the 
one  to  the  right  a  Spirillum  ;  B,  a  Coccus  form  (Planocoecus);  C,  D,  E,  species  of 
Pseudomonas :  F,  G,  species  of  Bacillus,  F  being  that  of  typhoid  fever;  H,  Micro- 
spira ;  J,  K,  L,  M,  species  of  Spirillum.— After  Englbr  and  Prantl. 


THALLOPHYTES:  FUNGI  77 

They  are  the  smallest  known  living  organisms,  the  one- 
celled  form  which  develops  on  cooked  potatoes,  bread,  milk, 
meat,  etc.,  forming  a  blood-red  stain,  having  a  diameter  of 
but  0.0005  mm.  (-^oioT  in.).  They  are  of  various  forms 
(Fig.  68),  as  Coccus  forms,  single  spherical  cells ;  Bacterium 
forms,  short  rod-shaped  cells ;  Bacillus  forms,  longer  rod- 
shaped  cells ;  Leptothrix  forms,  simple  filaments ;  Spirillum 
forms,  spiral  filaments,  etc. 

They  multiply  by  cell  division  with  wonderful  rapidity, 
and  also  form  resting  spores  for  preservation  and  distri- 
bution. They  occur  everywhere — in  the  air,  in  the  water, 
in  the  soil,  in  the  bodies  of  plants  and  animals ;  many  of 
them  harmless,  many  of  them  useful,  many  of  them  dan- 
gerous. 

They  are  intimately  concerned  with  fermentation  and 
decay,  inducing  such  changes  as  the  souring  of  fruit  juices, 
milk,  etc.,  and  the  development  of  pus  in  wounds.  What 
is  called  antiseptic  surgery  is  the  use  of  various  means  to 
exclude  bacteria  and  so  prevent  inflammation  and  decay. 

The  pathogenic  forms — that  is,  those  which  induce  dis- 
eases of  plants  and  animals — are  of  great  importance,  and 
means  of  making  them  harmless  or  destroying  them  are 
being  searched  for  constantly.  They  are  the  causes  of  such 
diseases  as  pear-blight  and  peach-yellows  among  plants,  and 
such  human  diseases  as  tuberculosis,  cholera,  diphtheria, 
typhoid  fever,  etc. 

LICHENS 

53.  General  character.  —  Lichens  are  abundant  every- 
where, forming  various  colored  splotches  on  tree-trunks, 
rocks,  old  boards,  etc.,  and  growing  also  upon  the  ground 
(Figs.  69,  70,  71).  They  have  a  general  greenish-gray  color, 
but  brighter  colors  may  also  be  observed. 

The  great  interest  connected  with  Lichens  is  that  they  are 
not  single  plants,  but  each  Lichen  is  formed  of  a  fungus  and 
an  alga,  living  together  so  intimately  as  to  appear  like  a  single 
24 


THALLOPHYTES :  FUNGI 


79 


plant.    In  other  words,  a  Lichen  is  not  an  individual,  but  a 
firm  of  two  individuals  very  unlike  each  other.     This  habit 


Fio.  70.    A  common  lichen  (Physcia)  growing  on  bark,  showing  the  spreading  thallus 
and  the  numerous  dark  disks  (apothecia)  bearing  the  asci.— Goi.dberger. 

of  living  together  has  been  called  symbiosis,  and  the  indi- 
viduals entering  into  this  relation  are  called  symbionts. 


Fig.  71.    A  common  foliose  lichen  {Parmdia)  growing  upon  a  board,  and  showing 
apothecia. — Goldberger. 


80 


PLANT   STRUCTURES 


If  a  Lichen  be  sectioned,  the  relation  between  the  sym- 
bionts  will  be  seen  (Fig.  72).  The  fungus  makes  the  bulk 
of  the  body  with  its  interwoven  mycelial  threads,  in  the 
meshes  of  which  lie  the  Algae,  sometimes  scattered,  some- 


'WBM 


Fig.  72.    Section  through  thallus  of  a  lichen  (Slicla),  showing  holdfasts  (r),  lower  (w) 
and  upper  (o)  surfaces,  fungus  hyphse  (m),  and  enmeshed  algae  (g). — After  Sachs. 


times  massed.  It  is  these  enmeshed  Algae,  showing  through 
the  transparent  mycelium,  that  give  the  greenish  tint  to 
the  Lichen. 

In  the  case  of  Lichens  the  symbionts  are  thought  by 
some  to  be  mutually  helpful,  the  alga  manufacturing  food 
for  the  fungus,  and  the  fungus  providing  protection  and 
water  containing  food  materials  for  the  alga.  Others  do  not 
recognize  any  special  benefit  to  the  alga,  and  see  in  a  Lichen 
simply  a  parasitic  fungus  living  on  the  products  of  an  alga. 
In  any  event  the  Algae  are  not  destroyed  but  seem  to  thrive. 
It  is  discovered  that  the  alga  symbiont  can  live  quite  inde- 


THALLOPHYTES:  FUNGI 


81 


pendently  of  the  fungus.  In  fact,  the  enmeshed  Algje  are 
often  recognized  as  identical  with  forms  living  independ- 
ently, those  thus  used  being  various  Blue-green,  Protococ- 
cus,  and  Conferva  forms  (see  p.  87). 

On  the  other  hand,  the  fungus  symbiont  has  become 
quite  dependent  upon  the  alga,  and  its  germinating  spores 
do  not  develop  far  unless  the  young  mycelium  can  lay  hold 
of  suitable  Algas.  At  certain  times  cup-like  or  disk-like 
bodies  appear  on  the  surface  of  the  lichen  thallus,  with 
brown,  or  black,  or  more  brightly-colored  lining  (Figs.  70, 
71).  These  bodies  are  the  apothecia,  and  a  section  through 
them  shows  that  the  colored  lining  is  largely  made  up  of 
delicate  sacs  containing  spores  (Figs.  73,  74).  These  sacs 
are  evidently  asci,  the  apothecia  correspond  to  ascocarps, 
and  the  Lichen  fungus  proves  to  be  an  Ascomycete. 


Fig.  73.  Section  through  an  apothecium  of  Anaptychia,  showing  stalk  of  the  cnp 
(to),  masses  of  algal  cells  (g),  outer  margin  of  cup  (?•),  overlapping  edge  (t,  t),  layer 
of  asci  (h),  and  massing  of  hyphs  beneath  asci  (y).— After  Sachs. 

Certain  Ascomycetes,  therefore,  have  learned  to  use  cer- 
tain Algae  in  this  peculiar  way,  and  a  Lichen  is  the  result. 
Some  Basidiomycetes  have  also  learned  the  same  habit,  and 
form  Lichens. 

Various  forms  of  Lichen  bodies  can  be  distinguished  as 
follows  :  (1)  Crustaceous  Lichens,  in  which  the  thallus  resem- 


82 


PLANT   STRUCTUKES 


bles  an  incrustation  upon  its  substratum  of  rock,  soil,  etc. ; 
(2)  Foliose  Lichens,  with  flattened,  leaf-like,  lobed  bodies,  at- 


Fig.  74.  Much  enlarged  section  of  a  portion  of  the  apothecium  of  Anaptychia,  show- 
ing the  fungus  mycelium  (m),  which  is  massed  above  (y),  just  beneath  the  layer  of 
asci  U,  2,  3,  It),  in  which  spores  in  various  stages  of  development  are  shown.— 
After  Sachs. 


tached  only  at  the  middle  or  irregularly  to  the  substratum  ; 
(3)  Fruticose  Lichens,  with  filamentous  bodies  branching 
like  shrubs,  either  erect,  pendulous,  or  prostrate. 


CHAPTER  VI 

THE    FOOD    OF   PLANTS 

54.  Introductory. — All  plants  use  the  same  kind  of  food, 
but  the  Algae  and  Fungi  suggest  that  they  may  have  very 
different  ways  of  obtaining  it.  The  Algss  can  manufacture 
food  from  raw  material,  while  the  Fungi  must  obtain  it 
already  manufactured.  Between  these  two  extreme  condi- 
tions there  are  plants  which  can  manufacture  food,  and  at 
the  same  time  have  formed  the  habit  of  supplementing  this 
by  obtaining  elsewhere  more  or  less  manufactured  food. 
Besides  this,  there  are  plants  which  have  learned  to  work 
together  in  the  matter  of  food  supply,  entering  into  a  con- 
dition of  symbiosis,  as  described  under  the  Lichens.  These 
various  habits  will  be  presented  here  briefly. 

55.  Green  plants. — The  presence  of  chlorophyll  enables 
plants  to  utilize  carbon  dioxide  (C02),  a  gas  present  in  the 
atmosphere  and  dissolved  in  waters,  and  one  of  the  waste 
products  given  off  in  the  respiration  of  all  living  organisms. 
This  gas  is  absorbed  by  green  plants,  its  constituent  ele- 
ments, carbon  and  oxygen,  are  dissociated,  and  with  the  ele- 
ments obtained  from  absorbed  water  (H20)  are  recombined  to 
form  a  carbohydrate  (sugar,  starch,  etc.),  which  is  an  organ- 
ized food.  With  this  as  a  basis  other  foods  are  formed, 
and  so  the  plant  can  live  without  help  from  any  other 
organism. 

This  process  of  utilizing  carbon  dioxide  in  the  formation 
of  food  is  not  only  a  wonderful  one,  but  also  very  important. 
It  is  wonderful,  because  carbon  dioxide  and  water,  both  of 
them  very  refractory  substances,  are  broken  up  at  ordinary 

83 


84  PLANT   STKUCTUKES 

temperatures  and  without  any  special  display  of  energy.  It 
is  important,  because  the  food  of  all  plants  and  animals  de- 
pends upon  it,  as  it  is  the  only  known  process  by  which  inor- 
ganic material  can  be  organized. 

The  process  is  called  photosynthesis,  or  photosyntaz, 
words  indicating  that  the  presence  of  light  is  necessary. 
The  mechanism  on  the  part  of  the  plant  is  the  chloroplast, 
which  when  exposed  to  light  is  able  to  do  this  work.  The 
process  is  often  called  "  carbon  assimilation,"  "  chlorophyll 
assimilation,"  "  fixation  of  carbon,"  etc.  It  should  be  noted 
that  it  is  not  the  chlorophyll  which  does  the  work,  but  the 
protoplasmic  plastid  stained  green  by  the  chlorophyll.  The 
chlorophyll  manipulates  the  light  in  some  way  so  that  the 
plastid  may  obtain  from  it  the  energy  needed  for  the  work. 
Further  details  concerning  it  may  be  obtained  by  reading 
§  112  of  Plant  Relations. 

It  is  evident  that  green  plants  must  expose  their  chloro- 
phyll to  the  light.  For  this  reason  the  Algae  can  not  live 
in  deep  waters  or  in  dark  places.  In  the  case  of  the  large 
marine  kelps,  although  they  may  be  anchored  in  considera- 
ble depth  of  water,  their  working  bodies  are  floated  up 
toward  the  light  by  air-bladders.  In  the  case  of  higher 
plants,  specially  organized  chlorophyll-bearing  organs,  the 
foliage  leaves,  are  developed. 

56.  Saprophytes. — Only  cells  containing  chloroplasts  can 
live  independently.  In  the  higher  plants,  where  bodies  be- 
come large,  many  living  cells  are  shut  away  from  the  light, 
and  must  depend  upon  the  more  superficial  green  cells  for 
their  food  supply.  The  habit  of  cells  depending  upon  one 
another  for  food,  therefore,  is  a  very  common  one. 

When  none  of  the  cells  of  the  plant  body  contain  chloro- 
phyll, the  whole  plant  becomes  dependent,  and  must  live  as 
a  saprophyte  or  a  parasite.  In  the  case  of  saprophytes  dead 
bodies  or  body  products  are  attacked,  and  sooner  or  later  all 
organic  matter  is  attacked  and  decomposed  by  them.  The 
decomposition  is  a  result  of  the  nutritive  processes  of  plants 


THE   FOOD   OF  PLANTS 


85 


without  chlorophyll,  and  were  it  not  for  them  "  the  whole 
surface  of  the  earth  would  be  covered  with  a  thick  deposit 
of  the  animal  and  plant  remains  of  the  past  thousands  of 
years." 

The  green  plants,  therefore,  are  the  manufacturers  of 
organic  material,  producing  far  more  than  they  can  use, 
while  the  plants  without  chlorophyll  are  the  destroyers  of 
organic  material.  The  chief  destroyers  are  the  Bacteria 
and  ordinary  Fungi,  but  some  of  the  higher  plants  have 
also  adopted  this  method  of  obtaining  food.  Many  ordinary 
green  plants  have  the  saprophytic  habit  of  absorbing  organic 
material  from  rich  humus  soil ;  and  some  plants  (as  broom 
rapes)  are  parasitic,  attaching  their  subterranean  parts  to 
those  of  other  plants, becoming  what  are  called  "root  para- 
sites." In  cases  of  mycorhiza  (see  p.  87),  which  are  now 
thought  to  include  great  numbers  of  green  plants,  it  is  sup- 
posed that  some  organic  material  is  brought  in  by  the  fungus. 

57.  Parasites. — Certain  plants  without  chlorophyll  are 
not  content  to  obtain  organic  material  from  dead  bodies, 
but  attack  living  ones.  As  in  the  case  of  saprophytes,  the 
vast  majority  of  plants  which  have  formed  this  habit  are 
Bacteria  and  ordinary  Fungi.  Parasites  are  not  only  modi- 
fied in  structure  in  consequence  of  the  absence  of  chloro- 
phyll, but  they  have  developed  means  of  penetrating  their 
hosts.  Many  of  them  have  also  cultivated  a  very  selective 
habit,  restricting  themselves  to  certain  plants  or  animals,  or 
even  to  certain  organs. 

The  parasitic  habit  has  also  been  developed  by  some  of 
the  higher  plants,  sometimes  completely,  sometimes  par- 
tially. Dodder,  for  example,  is  completely  parasitic  at 
maturity  (Fig.  75),  while  mistletoe  is  only  partially  so, 
doing  chlorophyll  work  and  also  absorbing  from  the  tree 
into  which  it  has  sent  its  haustoria. 

That  saprophytism  and  parasitism  are  both  habits  grad- 
ually acquired  is  inferred  from  the  number  of  green  plants 
which  have  developed  them  more  or  less,  as  a  supplement  to 


86 


PLANT   STRUCTURES 


the  food  which  they  manufacture.  The  less  chlorophyll  is 
used  the  less  is  it  developed,  and  a  green  plant  which  is 
obtaining  the  larger  amount  of  its  food  in  a  saprophytic 

or  parasitic  way  is 
on  the  way  to  losing 
all  of  its  chlorophyll 
and  becoming  a  com- 
plete saprophyte  or 
parasite. 

Certain  of  the  low- 
er Algae  are  in  the 
habit  of  living  in  the 
body  cavities  of  high- 
er plants,  finding  in 
such  situations  the 
moisture  and  protec- 
tion which  they  need. 
They  may  thus  have 
brought  within  their 
reach  some  of  the 
organic  products  of 
the  higher  plant.  If 
they  can  use  some  of 
these,  as  is  very  like- 
ly, a  partially  para- 
sitic habit  is  begun, 
which  may  lead  to 
loss  of  chlorophyll 
and  complete  para- 
sitism. 

58.    Symbionts.  — 
Symbiosis    means 
"  living      together," 
and  two  organisms  thus  related  are  called  symbionts.    In 
its  broadest  sense  symbiosis  includes  any  sort  of  depend- 
ence between  living  organisms,  from  the  vine  and  the  tree 


Fig.  75.  A  dodder  plant  parasitic  on  a  willow  twig. 
The  leafless  dodder  twines  about  the  willow,  and 
sends  out  sucking  processes  which  penetrate  and 
absorb. — After  Strasburger. 


THE  FOOD   OF  PLANTS  37 

upon  which  it  climbs,  to  the  alga  and  fungus  so  intimately 
associated  in  a  Lichen  as  to  seem  a  single  plant.  In  a  nar- 
rower sense  it  includes  only  cases  in  which  there  is  an  inti- 
mate organic  relation  between  the  symbionts.  This  would 
include  parasitism,  the  parasite  and  host  being  the  sym- 
bionts, and  the  organic  relation  certainly  being  intimate. 
In  a  still  narrower  sense  symbiosis  includes  only  those  cases 
in  which  the  symbionts  are  mutually  helpful.  Tbis  fact, 
however,  is  very  difficult  to  determine,  and  opinions  vary 
widely  as  to  the  mutual  advantage  in  certain  cases.  How- 
ever large  a  set  of  phenomena  may  be  included  under  the 
term  symbiosis,  we  use  it  here  in  this  narrowest  sense, 
which  is  often  distinguished  as  mutualism. 

(1)  Lichens. — A  lichen  is  a  complex  made  up  of  a  fun- 
gus and  an  alga  living  togetber.  It  is  certain  that  the  fun- 
gus cannot  live  without  the  alga,  but  the  alga  can  live 
without  the  fungus.  Hence  it  seems  plain  that  this  rela- 
tion is  not  one  of  mutual  helpfulness,  but  that  the  fungus 
is  living  upon  the  alga,  as  any  other  parasite  lives  upon  its 
host  (see  §  191). 

(2)  Mycorhiza. — The  name  means  "root-fungus,"  and 
refers  to  an  association  which  exists  between  certain  Fungi 
of  the  soil  and  roots  of  higher  plants.  It  was  formerly 
thought  that  mycorhiza  occurred  only  in  connection  with 
a  limited  number  of  higher  plants,  such  as  orchids,  heaths, 
oaks,  etc.,  but  more  recent  study  indicates  that  probably 
the  large  majority  of  vascular  plants  (that  is,  plants  with 
true  roots)  possess  it,  the  water  plants  being  excepted  (Figs. 
149,  150).  It  has  been  found  that  the  humus  soil  of  forests 
is  in  large  part  "  a  living  mass  of  innumerable  filamentous 
fungi."  It  is  clearly  of  advantage  to  roots  to  relate  them- 
selves to  this  great  network  of  filaments,  which  are  already 
in  the  best  relations  for  absorption,  and  those  plants  which 
are  unable  to  do  this  are  at  a  disadvantage  in  the  competi- 
tion for  the  nutrient  materials  of  the  forest  soil.  It  is 
doubtful  whether  many  vascular  green  plants  can  absorb 


Pig.  76.  Mycorhiza :  to  the  left  is  the  tip  of  a  rootlet  of  beech  enmeshed  by  the 
fungus;  A,  diagram  of  longitudinal  section  of  an  orchid  root,  showing  the  cells 
of  the  cortex  (p)  filled  with  hyphse;  B,  part  of  longitudinal  section  of  orchid  root 
much  enlarged,  showing  epidermis  (e),  outermost  cells  of  the  cortex  (p)  filled  with 
hyphal  threads,  which  are  sending  branches  into  the  adjacent  cortical  cells  (a,  i). 
—After  Frank. 


<*  8 

Ftg.  77.    Mycorhiza :  A,  rootlets  of  white  poplar  forming  mycorhiza  ;  B,  enlarged 

section  of  single  rootlets,  showing  the  hyphae  penetrating   the  cells. — After 

KlRNBB. 


THE  FOOD   OF  PLANTS 


89 


enough  for  their  needs  from  the  soil  without  this  assistance, 
and,  if  so,  the  fungus  becomes  of  vital  importance  in  the 
nutrition  of  such  plants.  In  the  case  of  some  of  these 
plants  it  seems  that  the  soil  fungus  is  not  merely  passing 
into  their  bodies  the  soil  water  with  its  dissolved  salts,  but 
is  contributing  to  them  organized  food,  thus  diminishing 
the  amount  of  necessary  food  manu- 
facture. The  delicate  branching  fila- 
ments (hyphae)  of  the  fungus  wrap 
the  rootlets  with  a  mesh  of  hyphae 
and  penetrate  into  the  cells,  and  it 
is  evident  that  the  fungus  obtains 
food  from  the  rootlet  as  a  parasite. 

(3)  Root-tubercles. — On  the  roots 
of  many  legume  plants,  as  clovers, 
peas,  beans,  etc.,  little  wart -like 
outgrowths  are  frequently  found, 
known  as  "  root-tubercles "  (Fig. 
78).  It  is  found  that  these  tuber- 
cles are  caused  by  certain  Bacteria, 
which  penetrate  the  roots  and  in- 
duce these  excrescent  growths.  The 
tubercles  are  found  to  swarm  with 
Bacteria,  which  are  doubtless  ob- 
taining food  from  the  roots  of  the 
host.  At  the  same  time,  these  Bac- 
teria have  the  peculiar  power  of 
laying  hold  of  the  free  nitrogen  of 
the  air  circulating  in  the  soil,  and 
of  supplying  it  to  the  host  plant 
in  some  usable  form.  Ordinarily 
plants  can  not  use  free  nitrogen, 

although  it  occurs  in  the  air  in  such  abundance,  and  this 
power  of  these  soil  Bacteria  is  peculiarly  interesting. 

This  habit  of  clover  and  its  allies  explains  why  they  are 
useful  in  what  is  called  "  restoring  the  soil."    After  ordi- 


Fig.  78.     Root-tubercles  on 
Yicia  Faba—  After  Noll. 


90 


PLANT   STRUCTURES 


nary  crops  have  exhausted  the  soil  of  its  nitrogen-contain- 
ing salts,  and  it  has  become  comparatively  sterile,  clover  is 
able  to  grow  by  obtaining  nitrogen  from  the  air  through  the 
root-tubercles.  If  the  crop  of  clover  be  "  plowed  under," 
nitrogen-containing  materials  which  the  clover  has  organ- 
ized will  be  contributed  to  the  soil,  which  is  thus  restored 
to  a  condition  which  will  support  the  ordinary  crops  again. 
This  indicates  the  significance  of  a  very  ordinary  "  rotation 
of  crops." 

(4)  Ant-plants,  etc. — In  symbiosis  one  of  the  symbionts 
may  be  an  animal.  Certain  fresh-water  polyps  and  sponges 
become  green  on  account  of  Algae  which  they  harbor  with- 
in their  bodies  (Fig.  79).  Like 
the  Lichen-fungus,  these  animals 
are  benefited  by  the  presence  of 
the  Algae,  which  in  turn  find  a 
congenial  situation  for  living. 
By  some  this  would  also  be  re- 
garded as  a  case  of  helotism, 
the  animal  enslaving  the  alga. 

Very  definite  arrangements 
are  made  by  certain  plants  for 
harboring  ants,  which  in  turn 
guard  them  against  the  attack 
of  leaf-cutting  insects  and  oth- 
er foes.  These  plants  are  called 
Myrmecophytes,  which  means 
"  ant-plants,"  or  myrmecophilous 
plants,  which  means  "plants  loving  ants."  These  plants 
are  mainly  in  the  tropics,  and  in  stem  cavities,  in  hollow 
thorns,  or  elsewhere,  they  provide  dwelling  places  for  tribes 
of  warlike  ants  (Fig.  80).  In  addition  to  these  dwelling 
places  they  provide  special  kinds  of  food  for  the  ants. 

(5)  Floivers  and  insects. — A  very  interesting  and  impor- 
tant case  of  symbiosis  is  that  existing  between  flowers  and 
insects.     The  flowers  furnish  food  to  the  insects,  and  the 


Fig.  79.  A  fresh-water  polyp  (Hy- 
dra) attached  to  a  twig  and  con- 
taining algse  (6'),  which  may  be 
seen  through  the  transparent 
body  wall  (B).— Goldberger. 


THE   FOOD    OF   PLANTS 


91 


latter  are  used  by  the  flowers  as  agents  of  pollination.     An 
account  of  this  relationship  is  deferred  until  seed-plants  are 


w&*iM 


Rv 


Fig.  80.    An  ant  plant  (Hydnophyturri)  from  South  Java,  in  which  an  excrescent 
growth  provides  a  habitation  for  ants.— After  Schimpek. 

considered,  or  it  may  be  found,  with  illustrations,  in  Plant 
Relations,  Chapter  VII. 


92  PLANT   STRUCTURES 

59.  Carnivorous  plants. — Certain  green  plants,  growing 
in  situations  poor  in  nitrogen-containing  salts,  have  learned 
to  supplement  the  proteids  which  they  manufacture  by  cap- 
turing and  digesting  insects.  The  various  devices  employed 
for  securing  insects  have  excited  great  interest,  since  they 
do  not  seem  to  be  associated  with  the  ordinary  idea  of  plant 
activities.  Prominent  among  these  forms  are  the  bladder- 
worts,  pitcher-plants,  sundews,  Venus's  fly-trap,  etc.  For 
further  account  and  illustrations  of  these  plants  see  Plant 
Relations,  §  119. 


CHAPTEE   VII 

BRYOPHYTES  (MOSS  PLANTS) 

60.  Summary  from  Thallophytes. — Before  considering  the 
second  great  division  of  plants  it  is  well  to  recall  the  most 
important  facts  connected  with  the  Thallophytes,  those 
things  which  may  be  regarded  as  the  contribution  of  the 
Thallophytes  to  the  evolution  of  the  plant  kingdom,  and 
which  are  in  the  background  when  one  enters  the  region  of 
the  Bryophytes. 

(1)  Increasing  complexity  of  the  tody. — Beginning  with 
single  isolated  cells,  the  plant  body  attains  considerable 
complexity,  in  the  form  of  simple  or  branching  filaments, 
cell-plates,  and  cell-masses. 

(2)  Appearance  of  spores. — The  setting  apart  of  repro- 
ductive cells,  known  as  spores,  as  distinct  from  nutritive 
cells,  and  of  reproductive  organs  to  organize  these  spores, 
represents  the  first  important  differentiation  of  the  plant 
body  into  nutritive  and  reproductive  regions. 

(3)  Differentiation  of  spores. — After  the  introduction  of 
spores  they  become  different  in  their  mode  of  origin,  but 
not  in  their  power.  The  asexual  spore,  ordinarily  formed 
by  cell  division,  is  followed  by  the  appearance  of  the  sexual 
spore,  formed  by  cell  union,  the  act  of  cell  union  being 
known  as  the  sexual  process. 

(4)  Differentiation  of  gametes.— At  the  first  appearance 
of  sex  the  sexual  cells  or  gametes  are  alike,  but  after- 
ward they  become  different  in  size  and  activity,  the  large 
passive  one  being  called  the  egg,  the  small  active  one  the 

25  98 


94  PLANT   STRUCTURES 

sperm,  the  organs  producing  the  two  being  known  as  oogo- 
nium and  antheridium  respectively. 

(5)  Algm  the  main  line. — The  Algae,  aquatic  in  habit, 
appear  to  be  the  Thallophytes  which  lead  to  the  Bryophytes 
and  higher  groups,  the  Fungi  being  regarded  as  their  de- 
generate descendants  ;  and  among  the  Algae  the  Chloro- 
phyceae  seem  to  be  most  probable  ancestors  of  higher  forms. 
It  should  be  remembered  that  among  these  Green  Algae  the 
ciliated  swimming  spore  (zoospore)  is  the  characteristic 
asexual  spore,  and  the  sexual  spore  (zygote  or  oospore)  is 
the  resting  stage  of  the  plant,  to  carry  it  over  from  one 
growing  season  to  the  next. 

61.  General  characters  of  Bryophytes. — The  name  given 
to  the  group  means  "  moss  plants,"  and  the  Mosses  may  be 
regarded  as  the  most  representative  forms.  Associated 
with  them  in  the  group,  however,  are  the  Liverworts,  and 
these  two  groups  are  plainly  distinguished  from  the  Thallo- 
phytes below,  and  from  the  Pteridophytes  above.  Starting 
with  the  structures  that  the  Algae  have  worked  out,  the 
Bryophytes  modify  them  still  further,  and  make  their  own 
contributions  to  the  evolution  of  the  plant  kingdom,  so 
that  Bryophytes  become  much  more  complex  than  Thallo- 
phytes. 

62.  Alternation  of  generations. — Probably  the  most  im- 
portant fact  connected  with  the  Bryophytes  is  the  distinct 
alternation  of  generations  which  they  exhibit.  So  impor- 
tant is  this  fact  in  connection  with  the  development  of  the 
plant  kingdom  that  its  general  nature  must  be  clearly  under- 
stood. Probably  the  clearest  definition  may  be  obtained  by 
tracing  in  bare  outline  the  life  history  of  an  ordinary  moss. 

Beginning  with  the  asexual  spore,  which  is  not  ciliated, 
as  there  is  no  water  in  which  it  can  swim,  we  may  imagine 
that  it  has  been  carried  by  the  wind  to  some  spot  suitable 
for  its  germination.  It  develops  a  branching  filamentous 
growth  which  resembles  some  of  the  Conferva  forms  among 
the  Green  Algae  (Fig.  81).     It  is  prostrate,  and  is  a  regu- 


BRYOPHYTES 


95 


lar  thallus  body,  not  at  all  resembling  the  "  moss  plant " 
of  ordinary  observation,  and  is  not  noticed  by  those  una- 
ware of  its  existence. 

Presently  one  or  more  buds  appear  on  the  sides  of  this 
alga-like  body  (Fig.  81,  h).     A  bud  develops  into  an  erect 


Pig.  81.  Protonema  of  moss :  A,  very  young  protonema,  showing  spore  (S)  which 
has  germinated  it;  B,  older  protonema,  showing  branching  habit,  remains  of 
spore  («),  rhizoids  (r),  and  buds  (6)  of  leafy  branches  (gametophores).— After 
Muller  and  Thurgau. 


stalk  upon  which  are  numerous  small  leaves  (Figs.  82, 102). 
This  leafy  stalk  is  the  "  moss  plant "  of  ordinary  observa- 
tion, and  it  will  be  noticed  that  it  is  simply  an  erect  leafy 
branch  from  the  prostrate  alga-like  body. 

At  the  top  of  this  leafy  branch  sex-organs  appear,  cor- 
responding to  the  antheridia  and  oogonia  of  the  Algae,  and 
within  them  there  are  sperms  and  eggs.  A  sperm  and  egg 
fuse  and  an  oospore  is  formed  at  the  summit  of  the  leafy 
branch. 

The  oospore  is  not  a  resting  spore,  but  germinates  im- 
mediately, forming  a  structure  entirely  unlike  the  moss 


96 


PLANT  STKDCTUEES 


,rh 


Pig.  82.  A  common  mos8 
(Po lytrichu m  commune), 
showing  the  leafy  gameto- 
phore  with  rhizoids  (rA), 
and  two  sporophytes  (sporo- 
gonia),  with  seta  (s),  calyp- 
tra  (c),  and  operculum  (d), 
the  calyptra  having  been  re- 
moved.—After  Schenck. 


plant  from  which  it  came.  This  new 
leafless  body  consists  of  a  slender 
stalk  bearing  at  its  summit  an  urn- 
like case  in  which  are  developed  nu- 
merous asexual  spores  (Figs.  82, 107). 
This  whole  structure  is  often  called 
the  "spore  fruit,"  and  its  stalk  is 
imbedded  at  base  in  the  summit  of 
the  leafy  branch,  thus  obtaining  firm 
anchorage  and  absorbing  what  nour- 
ishment it  needs,  but  no  more  a  part 
of  the  leafy  branch  than  is  a  para- 
site a  part  of  the  host. 

When  the  asexual  spores,  pro- 
duced by  the  "spore  fruit,"  germi- 
nate, they  reproduce  the  alga-like 
body  with  which  we  began,  and  the 
life  cycle  is  completed. 

In  examining  this  life  history,  it 
is  apparent  that  each  spore  produces 
a  different  structure.  The  asexual 
spore  produces  the  alga-like  body 
with  its  erect  leafy  branch,  while 
the  oospore  produces  the  "  spore 
fruit"  with  its  leafless  stalk  and 
spore  case.  These  two  structures, 
one  produced  by  the  asexual  spore, 
the  other  by  the  oospore,  appear  in 
alternating  succession,  and  this  is 
what  is  meant  by  alternation  of  gen- 
erations. 

These  two  "generations"  differ 
strikingly  from  one  another  in  the 
spores  which  they  produce.  The 
generation  composed  of  alga -like 
body  and   erect   leafy  branch  pro- 


BRYOPHYTES 


97 


duces  only  sexual  spores  (oospores),  and  therefore  pro- 
duces sex  organs  and  gametes.  It  is  known,  therefore, 
as  the  gametophyte — that  is,  "the  gamete  plant." 

The  generation  which  consists  of  the  "spore  fruit" — 
that  is,  leafless  stalk  and  spore  case — produces  only  asexual 
spores,  and  is  called  the  sporophyte — that  is,  "the  spore 
plant." 

Alternation  of  generations,  therefore,  means  the  alter- 
nation of  a  gametophyte  and  a  sporophyte  in  completing  a 
life  history.  Instead  of  having  the  same  body  produce  both 
asexual  and  sexual  spores,  as  in  most  of  the  Algae,  the  two 
kinds  of  spores  are  separated  upon  different  structures, 
known  as  "generations."  It  is  evident  that  the  gameto- 
phyte is  the  sexual  generation,  and  the  sporophyte  the 
asexual  one ;  and  it  should  be  kept  clearly  in  mind  that 
the  asexual  spore  always  produces  the  gametophyte,  and 
the  sexual  spore  the  sporophyte.  In  other  words,  each 
spore  produces  not  its  own  generation,  but  the  other  one. 

The  relation  between  the  two  alternating  generations 
may  be  indicated  clearly  by  the  following  formula,  in 
which  Gr  and  S  are  used  for  gametophyte  and  sporophyte 
respectively : 

G=g>o— S— o— G=g>o— S— o— G,  etc. 

The  formula  indicates  that  the  gametophyte  produces 
two  gametes  (sperm  and  egg),  which  fuse  to  form  an  oospore, 
which  produces  the  sporophyte,  which  produces  an  asexual 
spore,  which  produces  a  gametophyte,  etc. 

That  alternation  of  generations  is  of  great  advantage  is 
evidenced  by  the  fact  that  it  appears  in  all  higher  plants. 
It  must  not  be  supposed  that  it  appears  first  in  the  Bryo- 
phytes,  for  its  beginnings  may  be  seen  among  the  Thallo- 
phytes.  The  Bryophytes,  however,  first  display  it  fully 
organized  and  without  exception.  Just  what  this  alterna- 
tion does  for  plants  may  not  be  fully  known,  but  one 
advantage  seems  prominent.  By  means  of  it  many  gameto- 
phytes  may  result  from  a  single  oospore ;  in  other  words, 


98  PLANT   STRUCTURES 

it  multiplies  the  product  of  the  sexual  spore.  A  glance  at 
the  formula  given  above  shows  that  if  there  were  no  sporo- 
phyte  (S)  the  oospore  would  produce  but  one  gametophyte 
(G).  By  introducing  the  sporophyte,  however,  as  many 
gametophytes  may  result  from  a  single  oospore  as  there  are 
asexual  spores  produced  by  the  sporophyte,  which  usually 
produces  a  very  great  number. 

In  reference  to  the  sporophytes  and  gametophytes  of 
Bryophytes  two  peculiarities  may  be  mentioned  at  this 
point :  (1)  the  sporophyte  is  dependent  upon  the  gameto- 
phyte for  its  nourishment,  and  remains  attached  to  it ; 
(2)  the  gametophyte  is  the  special  chlorophyll -generation, 
and  hence  is  the  more  conspicuous.  It  follows  that,  in  a 
general  way,  the  sporophyte  of  the  Bryophytes  only  pro- 
duces spores,  while  the  gametophyte  both  produces  gametes 
and  does  chlorophyll  work. 

It  is  important  also  to  note  that  the  protected  resting 
stage  in  the  life  history  is  not  the  sexual  spore,  as  in  the 
Algae,  but  is  the  asexual  spore  in  connection  with  the 
sporophyte.  These  spores  have  a  protecting  wall,  are 
scattered,  and  may  remain  for  some  time  without  germi- 
nation. 

If  the  ordinary  terms  in  reference  to  Mosses  be  fitted 
to  the  facts  given  above,  it  is  evident  that  the  "moss 
plant "  is  the  leafy  branch  of  the  gametophyte  ;  that 
the  "  moss  fruit "  is  the  sporophyte ;  and  that  the  alga- 
like part  of  the  gametophyte  has  escaped  attention  and 
a  popular  name. 

The  names  now  given  to  the  different  structures  which 
appear  in  this  life  history  are  as  follows  :  The  alga-like  part 
of  the  gametophyte  is  the  protonema,  the  leafy  branch  is 
the  gametophore  ("gamete-bearer") ;  the  whole  sporophyte 
is  the  sporogonium  (a  name  given  to  this  peculiar  leafless 
sporophyte  of  Bryophytes),  the  stalk-like  portion  is  the 
seta,  the  part  of  it  imbedded  in  the  gametophore  is  the 
foot,  and  the  urn-like  spore-case  is  the  capsule. 


BKYOPHYTES 


99 


63.  The  antheridium. — The  male  organ  of  the  Bryophytes 
is  called  an  antheridium,  just  as  among  Thallophytes,  but 
it  has  a  very  different  structure.     In  general  among  the 


Fig.  83.  Sex  organs  of  a  common  moss  (Funaria):  the  group  to  the  right  represents 
an  antheridium  (A)  discharging  from  its  apes  a  mass  of  sperm  mother  cells  (a),  a 
single  mother  cell  with  its  sperm  (b),  and  a  single  sperm  (c),  showing  body  and 
two  cilia;  the  group  to  the  left  represents  an  archegonial  cluster  at  summit  of 
stem  (A),  showing  archegonia  (a),  and  paraphyses  and  leaf  sections  (b),  and  also  a 
single  archegonium  IB),  with  venter  (b)  containing  egg  and  ventral  canal  cell,  and 
neck  (A)  containing  the  disorganizing  axial  row  (neck  canal  cells).— After  Sach6. 

Thallophytes  it  is  a  single  cell  (mother  cell),  and  may  be 
called  a  simple  antheridium,  but  in  the  Bryophytes  it  is  a 
many-celled  organ,  and  may  be  regarded  as  a  compound 
antheridium.    It  is  usually  a  stalked,  club-shaped,  or  oval  to 


100 


PLANT   STRUCTURES 


globular  body  (Figs.  83,  84,  103).  A  section  through  this 
body  shows  it  to  consist  of  a  single  layer  of  cells,  which 
forms  the  wall  of  the  antheridium,  and  within  this  a  com- 
pact mass  of  small  cubical  (square  in  section)  cells,  within 
each  one  of  which  there  is  formed  a  single  sperm  (Fig.  84). 
These  cubical  cells  are  evidently  moth- 
er cells,  and  to  distinguish  them  from 
others  they  are  called  sperm  mother  cells. 
An  antheridium,  therefore,  aside  from 
its  stalk,  is  a  mass  of  sperm  mother 
cells  surrounded  by  a  wall  consisting 
of  one  layer  of  cells. 

The  sperm  is  a  very  small  cell  with 
two  long  cilia  (Fig.  83).  The  two 
parts  are  spoken  of  as  "body"  and 
cilia,  and  the  body  may  be  straight  or 
somewhat  curved.  These  small  bicili- 
ate  sperms  are  one  of  the  distinguish- 
ing marks  of  the  Bryophytes.  The 
existence  of  male  gametes  in  the  form 
of  ciliated  sperms  indicates  that  fertil- 
ization can  take  place  only  in  the  pres- 
ence of  water,  so  that  while  the  plant 
has  become  terrestrial,  and  its  asexual  spores  have  respond- 
ed to  the  new  conditions  and  are  no  longer  ciliated,  its 
sexual  process  is  conducted  as  among  the  Green  Algae.  It 
must  not  be  supposed,  however,  that  any  great  amount  of 
water  is  necessary  to  enable  sperms  to  swim,  even  a  film 
of  dew  often  answering  the  purpose. 

When  the  mature  antheridia  are  wet  they  are  opened 
at  the  apex  and  discharge  the  mother  cells  in  a  mass  (Figs. 
83,  105,  E),  the  walls  of  the  mother  cells  become  mucilagi- 
nous, and  the  sperms  escaping  swim  actively  about  and  are 
attracted  to  the  organ  containing  the  egg. 

64.  The  archegonium. — This  name  is  given  to  the  female 
sex  organ,  and  it  is  very  different  from  the  oogonium  of 


Fig.  84.  Antheridium  of 
a  liverwort  in  section, 
showing  single  layer 
of  wall  cells  surround- 
ing the  mass  of  moth- 
er cells. — After  Stras- 

BURGER. 


BEYOPHYTES  101 

Thallophytes.  Instead  of  being  a  single  mother  cell,  it  is 
a  many-celled  structure,  shaped  like  a  flask  (Figs.  83,  98). 
The  neck  of  the  flask  is  more  or  less  elongated,  and  within 
the  bulbous  base  (venter)  the  single  egg  is  organized.  The 
archegonium,  made  up  of  neck  and  venter,  consists  mostly 
of  a  single  layer  of  cells.  This  hollow  flask  is  solid  at  first, 
there  being  a  central  vertical  row  of  cells  surrounded  by 
the  single  layer  just  referred  to.  All  of  the  cells  of  this 
axial  row,  except  the  lowest  one,  disorganize  and  leave  a 
passageway  down  through  the  neck.  The  lowest  one  of 
the  row,  which  lies  in  the  venter  of  the  archegonium,  or- 
ganizes the  egg.  In  this  way  there  is  formed  in  the  arche- 
gonium an  open  passageway  through  the  neck  to  the  egg 
lying  in  the  venter. 

To  this  neck  the  swimming  sperms  are  attracted,  enter 
and  pass  down  it,  one  of  them  fuses  with  the  egg,  and  this 
act  of  fertilization  results  in  an  oospore. 

Archegonia  and  antheridia  are  supposed  to  have  been 
derived  from  a  many-celled  gametangium,  such  as  occurs 
in  certain  Brown  Algae  (Fig.  18).  The  presence  of  the 
archegonia  is  one  strong  and  unvarying  distinction  between 
Thallophytes  and  Bryophytes.  Pteridophytes  also  have 
archegonia,  and  so  characteristic  an  organ  is  it  that  Bryo- 
phytes and  Pteridophytes  are  spoken  of  together  as  Arche- 
goniates. 

65.  Germination  of  the  oospore. — The  oospore  in  Bryo- 
phytes is  not  a  resting  spore,  but  germinates  immediately 
by  cell  division,  forming  the  sporophyte  embryo,  which 
presently  develops  into  the  mature  sporophyte  (Fig.  85,^4). 
The  lower  part  of  the  embryo  develops  the  foot,  which  ob- 
tains a  firm  anchorage  in  the  gametophore  by  the  latter 
growing  up  around  it  (Fig.  85,  2?,  C).  The  upper  part  of 
the  embryo  develops  the  seta  and  capsule.  As  the  embryo 
increases  in  size,  the  venter  of  the  archegonium  grows  also, 
forming  what  is  called  the  calyptra ;  and  in  true  Mosses 
the  embryo  presently  breaks  loose  the  calyptra  at  its  base 


102 


PLANT  STKUCTUEES 


and  carries  it  upward  perched  on  the  top  of  the  capsule  like 
a  loose  cap  or  hood  (Figs.  82,  c,  107),  which  sooner  or  later 

falls  off.  As  stated  be- 
fore, the  mature  struc- 
ture developed  from 
the  oospore  is  called  a 
sporogonium,  a  form  of 
sporophyte  peculiar  to 
the  Bryophytes. 

66.  The  sporogonium. 
— In  its  fullest  devel- 
ojjment  the  sporogoni- 
um is  differentiated 
into  the  three  regions, 
foot,  seta,  and  capsule 
(Figs.  82,  107)  ;  but  in 
some  forms  the  seta 
may  be  lacking,  and 
in  others  the  foot  also, 
the  sporogonium  in  this 
last  case  being  only  the 
capsule  or  spore  case, 
which,  after  all,  is  the 
essential  part  of  any 
sporogonium. 

At  first  the  capsule 
is  solid,  and  its  cells 
are  all  alike.  Later  a 
group  of  cells  within 
begins  to  differ  in  ap- 
pearance from  those 
about  them,  being  set 
apart  for  the  produc- 
tion of  spores.  This 
initial  group  of  spore-producing  cells  is  called  the  arclie- 
sporium,  a  word  meaning  "the  beginning  of  spores."    It 


Fig.  85.  Sporogonium  of  Fvnaria:  A,  an  em- 
bryo sporogonium  (/,/'),  developing  within 
the  venter  (b,  b)  of  an  archegonium ;  B,  C, 
tips  of  leafy  shoots  bearing  young  eporo- 
gonia,  pushing  up  calyptra  (c)  and  archego- 
nium neck  (h),  and  sending  the  foot  down 
into  the  apex  of  the  gametophore. — After 
Goebel. 


BKYOPHYTES  103 

does  not  follow  that  the  archesporial  cells  themselves  pro- 
duce spores,  but  that  the  spores  are  to  appear  sooner  or 
later  in  their  progeny.  Usually  the  archesporial  cells 
divide  and  form  a  larger  mass  of  spore-producing  cells. 
Such  cells  are  known  as  sporogenous  ("spore-producing") 
cells,  or  the  group  is  spoken  of  as  sporogenous  tissue.  Spo- 
rogenous cells  may  divide  more  or  less,  and  the  cells  of  the 
last  division  are  mother  cells,  those  which  directly  produce 
the  spores.  The  usual  sequence,  therefore,  is  archesporial 
cells  (archesporium),  sporogenous  cells,  and  mother  cells ; 
but  it  must  be  remembered  that  they  all  may  be  referred 
to  as  sporogenous  cells. 

Each  mother  cell  organizes  within  itself  four  spores, 
the  group  being  known  as  a  tetrad.  In  Bryophytes  and 
the  higher  groups  asexual  spores  are  always  produced  in 
tetrads.  After  the  spores  are  formed  the  walls  of  the 
mother  cells  disorganize,  and  the  spores  are  left  lying  loose 
in  a  cavity  which  was  formerly  occupied  by  the  sporoge- 
nous tissue.  All  mother  cells  do  not  always  organize  spores. 
In  some  cases  some  of  them  are  used  up  in  supplying  nour- 
ishment to  those  which  form  spores.  Such  mother  cells  are 
said  to  function  as  nutritive  cells.  In  other  cases,  certain 
mother  cells  become  much  modified  in  form,  being  organ- 
ized into  elongated,  spirally-banded  cells  called  elaters  (Figs. 
97,  101),  meaning  "drivers"  or  "hurlers."  These  elaters 
lie  among  the  loose  ripe  spores,  are  discharged  with  them, 
and  by  their  jerking  movements  assist  in  scattering  them. 

The  cells  of  the  sporogonium  which  do  not  enter  into 
the  formation  of  the  archesporium,  and  are  not  sporoge- 
nous, are  said  to  be  sterile,  and  are  often  spoken  of  as 
sterile  tissue.  Every  sporogonium,  therefore,  is  made  up 
of  sporogenous  tissue  and  sterile  tissue,  and  the  differences 
found  among  the  sporogonia  of  Bryophytes  depend  upon 
the  relative  display  of  these  two  tissues. 

The  sporogonium  is  a  very  important  structure  from 
the  standpoint  of  evolution,  for  it  represents  the  conspicu- 


104 


PLANT   STKUCTUKES 


ous  part  of  the  higher  plants.  The  "fern  plant,"  and 
the  herbs,  shrubs,  and  trees  among  "  flowering  plants " 
correspond  to  the  sporogonium  of  Bryophytes,  and  not  to 
the  leafy  branch  (gametophore)  or  "moss  plant."  Conse- 
quently the  evolution  of  the  sporogonium  through  the 
Bryophytes  is  traced  with  a  great  deal  of  interest.  It  may 
be  outlined  as  follows  : 

In  a  liverwort  called  Riccia  the  simplest  sporogonium 
is  found.     It  is  a  globular  capsule,  without  seta  or  foot 


Pig.  86.  Diagrammatic  sections  of  sporogonia  of  liverworts:  A.  Riccia,  the  whole 
capsule  being  archesporium  except  the  sterile  wall  layer;  B,  Marchantia,  one 
half  the  capsule  being  sterile,  the  archesporium  restricted  to  the  other  half;  D, 
Anthoceros,  archesporium  still  more  restricted,  being  dome-shaped  and  capping  a 
central  sterile  tissue,  the  columella  (col).—  After  Goebel. 


(Fig.  86,  A).  The  only  sterile  tissue  is  the  single  layer  of 
cells  forming  the  wall,  all  the  cells  within  the  wall  be- 
longing to  the  archesporium.  The  ripe  sporogonium, 
therefore,  is  nothing  but  a  thin-walled  spore  case.  It  is 
well  to  note  that  the  sporophyte  thus  begins  as  a  spore 
case,  and*  that  any  additional  structures  that  it  may  de- 
velop later  are  secondary. 

In  another  liverwort  (Marchantia)  the  entire  lower  half 
of  the  sporogonium  is  sterile,  while  in  the  upper  half  there 


BKYOrHYTES 


105 


is  a  single  layer  of  sterile  cells  as  a  wall  about  the  arche- 
sporium,  which  is  composed  of  all  the  remaining  cells  of  the 
upper  half  (Fig.  86,  B).  It  will  be  noted  that  the  sterile 
tissue  in  this  sporogonium  has  encroached  upon  the  arche- 
sporium, which  is  restricted  to  one  half  of  the  body.  In 
this  case  the  archesporium  has  the  form  of  a  hemisphere. 

In  another  liverwort  (Jungermannia)  the  archesporium 
is  still  more  restricted  (Fig.  87).    The  sterile  tissue  is  organ- 


Fig.  87.  Diagrammatic  section  of  spo- 
rogonium of  a  Jungermannia  form, 
showing  differentiation  into  foot, 
seta,  and  capsule,  the  archesporium 
restricted  to  upper  part  of  sporogo- 
nium.—After  Goebel. 


Fig.  88.  Section  through  sporogonium  of 
Sphagnum,  showing  capsule  (k)  with 
old  archegonium  neck  (ah),  calyptra  (ca), 
dome-shaped  mass  of  sporogenous  tissue 
(spo),  and  columella  (co),  also  the  bulb- 
ous foot  (spf)  imbedded  in  the  pseudo- 
podium  (ps).—  After  Schimper. 


ized  into  a  foot  and  a  seta,  and  the  archesporium  is  a  com- 
paratively small  mass  of  cells  in  the  upper  part  of  the 
sporogonium. 

In  another  liverwort  (Anthoceros)  the  sterile  tissue  or- 
ganizes foot  and  seta,  and  the  archesporium  is  still  more 
restricted  (Fig.  86,  D).     Instead  of  a  solid  hemispherical 


106 


PLANT   STEUCTUEES 


mass,  it  is  a  dome-shaped  mass,  the  inner  cells  of  the  hemi- 
sphere having  become  sterile.  This  central  group  of  sterile 
cells  which  is  surrounded  by  the  ar- 
chesporium  is  called  the  columella, 
which  means  "a  small  column." 

In  a  moss  called  Sphagnum  there 
is  the  same  dome-shaped  archespori- 
um  with  the  columella,  as  in  An- 
thoceros,  but  it  is  relatively  smaller 
on  account  of  the  more  abundant 
sterile  tissue  (Fig.  88). 

In  the  highest  Mosses  the  arche- 
sporium  becomes  very  small  as  com- 
pared with  the  sterile  tissue  (Fig. 
89).  A  foot,  a  long  seta,  and  an 
elaborate  capsule  are  organized  from 
the  sterile  tissue,  while  the  arche- 
sporium  is  shaped  like  the  walls  of 
a  barrel,  as  though  the  dome-shaped 
archesporium  of  Sphagnum  or  An- 
thoceros  had  become  sterile  at  the 
apex.  In  this  way  the  columella  is 
continued  through  the  capsule,  and 
is  not  capped  by  the  archesporium. 

This  series  indicates  that  after 
the  sporogonium  begins  as  a  simple 
spore  case  {Riccia),  its  tendency  is 
to  increase  sterile  tissue  and  to  re- 
strict sporogenous  tissue,  using  the 
sterile  tissue  in  the  formation  of  the 
organs  of  the  sporogonium  body,  as 
foot,  seta,  capsule  walls,  etc. 

Among  the  Green  Algae  there  is 
a  form  known  as  Coleoclmte,  whose 
body  resembles  those  of  the  sim- 
plest Liverworts   (Fig.  90).     When 


Fig.  89.  Young  sporogoni- 
um of  a  true  moss,  show- 
ing foot,  seta,  and  young 
capsule,  in  which  the  ar- 
chesporium (darker  por- 
tion) is  barrel-shaped,  and 
through  it  the  columella  is 
continuous  with  the  lid. — 
After  Campbell. 


BRYOPHYTES  107 

its  oospores  germinate  there  is  formed  a  globular  mass  of 
cells,  every  one  of  which  is  a  spore  mother  cell  (Fig.  90,  C). 
If  an  outer  layer  of  mother  cells  should  become  sterile  and 
form  a  wall  about  the  others,  such  a  spore  case  as  that  of 


Fig.  90.— Coleochate,  one  of  the  green  algse :  A,  a  portion  of  the  thallus,  showing 
oogonia  with  trichogynes  (og),  antheridia  (an),  and  two  enlarged  biciliate  sperms 
(z);  B,  a  fertilized  oogonium  containing  oospore  and  invested  by  a  tissue  (r) 
which  has  developed  after  fertilization  ;  C,  an  oospore  which  has  germinated 
and  formed  a  mass  of  cells  (probably  a  sporophyte),  each  one  of  which  organizes 
a  biciliate  zoospore  (Z>).— After  Pringsheim. 


Riccia  would  be  the  result  (Fig.  86,  A).  For  such  reasons 
many  believe  that  the  Liverworts  have  been  derived  from 
such  forms  as  Coleochcete. 

67.  The  gametophyte. — Having  considered  the  sporo- 
phyte body  as  represented  by  the  sporogonium,  we  must 
consider  the  gametophyte  body  as  represented  by  proto- 
nema  and  leafy  branch  (gametophore).  The  gametophyte 
results  from  the  germination  of  an  asexual  spore,  and  in 
the  Mosses  it  is  differentiated  into  protonema  and  leafy 
gametophore  (Figs.  81,  82,  102).      Like   the  sporophyte, 


IQg  PLANT   STKUCTUKES 

however,  it  shows  an  interesting  evolution  from  its  sim- 
plest condition  in  the  Liverworts  to  its  most  complex  con- 
dition in  the  true  Mosses. 

In  the  Liverworts  the  spore  develops  a  flat  thallus  body, 
one  plate  of  cells  or  more  in  thickness,  which  generally 
branches  dichotomously  (see  §  29)  and  forms  a  more  or  less 
extensive  body  (Fig.  92).  This  thallus  is  the  gametophyte, 
there  being  no  differentiation  into  protonema  and  leafy 
branch. 

In  the  simpler  Liverworts  the  sex  organs  (antheridia 
and  archegonia)  are  scattered  over  the  back  of  this  thallus 
(Fig.  92).  In  other  forms  they  become  collected  in  certain 
definite  regions  of  the  thallus.  In  other  forms  these  defi- 
nite sexual  regions  become  differentiated  from  the  rest  of 
the  thallus  as  disks.  In  other  forms  these  disks,  bearing 
the  sex  organs,  become  short-stalked,  and  in  others  long- 
stalked,  until  a  regular  branch  arises  from  the  thallus 
body  (Figs.  96,  97).  This  erect  branch,  bearing  the  sex  or- 
gans, is,  of  course,  a  gametophore,  but  it  is  leafless,  the 
thallus  body  doing  the  chlorophyll  work. 

In  the  Sphagnum  Mosses  the  spore  develops  the  same 
kind  of  flat  thallus  (Fig.  104),  but  the  gametophore  be- 
comes leafy,  sharing  the  chlorophyll  work  with  the  thallus. 
In  the  true  Mosses  most  of  the  chlorophyll  work  is  done  by 
the  leafy  gametophore,  and  the  flat  thallus  is  reduced  to 
branching  filaments  (the  protonema)  (Fig.  102). 

The  protonema  of  the  true  Mosses,  therefore,  corre- 
sponds to  the  flat  thallus  of  the  Liverworts  and  Sphagnum, 
while  the  leafy  branch  corresponds  to  the  leafless  gameto- 
phore found  in  some  Liverworts.  It  also  seems  evident 
that  the  gametophore  was  originally  set  apart  to  bear  sex 
organs,  and  that  the  leaves  which  appear  upon  it  in  the 
Mosses  are  subsequent  structures. 


CHAPTER   VIII 

THE  GREAT  GROUPS  OF  BRYOPHYTES 

Hepatic^e  {Liverworts) 

68.  General  character. — Liverworts  live  in  a  variety  of 
conditions,  some  floating  on  the  water,  many  in  damp 
places,  and  many  on  the  bark  of  trees.  In  general  they  are 
moisture-loving  plants  (hydrophytes),  though  some  can  en- 
dure great  dryness.  The  gametophyte  body  is  prostrate, 
though  there  may  be  erect  and  leafless  gametophores. 

This  prostrate  habit  develops  a  dorsiventral  body — that 
is,  one  whose  two  surfaces  {dorsal  and  ventral)  are  exposed 
to  different  conditions  and  become  unlike  in  structure.  In 
Liverworts  the  ventral  surface  is  against  the  substratum, 
and  puts  out  hair-like  processes  {rhizoids)  for  anchorage 
and  possibly  absorption.  The  dorsal  region  is  exposed  to 
the  light  and  its  cells  develop  chlorophyll.  If  the  thallus 
is  thin,  chlorophyll  is  developed  in  all  the  cells ;  if  it  be  so 
thick  that  the  light  is  cut  off  from  the  ventral  cells,  the 
thallus  is  differentiated  into  a  green  dorsal  region  doing  the 
chlorophyll  work,  and  a  colorless  ventral  region  producing 
anchoring  rhizoids.  This  latter  represents  a  simple  differ- 
entiation of  the  nutritive  body  into  working  regions,  the 
ventral  region  absorbing  material  and  conducting  it  to  the 
green  dorsal  cells  which  use  it  in  making  food. 

There  seem  to  have  been  at  least  three  main  lines  of 
development  among  Liverworts,  each  beginning  in  forms 
with  a  very  simple  thallus,  and  developing  in  different  di- 
rections.    They  are  briefly  indicated  as  follows : 
26  109 


110 


PLANT   STRUCTURES 


69.  Marchantia  forms. — In  this  line  the  simple  thallus 
gradually  becomes  changed  into  a  very  complex  one.     The 

thallus  retains  its  simple 
outlines,  but  becomes  thick 
and  differentiated  in  tissues 
(groups  of  similar  cells). 
The  line  may  be  distin- 
guished, therefore,  as  one 
in  which  the  differentia- 
tion of  the  tissues  of  the 
gametophyte  is  emphasized 
(Figs.  91-93).  In  Mar- 
chantia proper  the  thallus 
becomes  very  complex,  and 
it  may  be  taken  as  an  illus- 
tration. 

The  thallus  is  so  thick 

that  there  are  very  distinct 

green   dorsal   and  colorless 

ventral  regions  (Fig.  94).     The  latter  puts  out  numerous 

rhizoids  and  scales  from  the  single  layer  of  epidermal  cells. 

Above  the  ventral  epidermis  are  several  layers  of  colorless 


Fig.  91.  A  very  small  species  of  Jticcia, 
one  of  the  Marchantia  forms  :  A,  a 
group  of  thallus  bodies  slightly  en- 
larged ;  J3,  section  of  a  thallus,  show- 
ing rhizoids  and  two  sporogonia  im- 
bedded and  communicating  with  the 
outside  by  tubular  passages  in  the 
thallus. — After  Stbasburger. 


Fig.  92.  Ricciocarpus,  a  Marchantia  form,  showing  numerous  rhizoids  from  ventral 
surface,  the  dichotomous  branching,  and  the  position  of  the  sporogonia  on  the 
dorsal  surface  along  the  "midribs."— Goldberger. 


Fig.  93.  Two  common  liverworts  :  to  the  left  is  Conocephalus,  a  Marchantia  form, 
showing  rhizoids,  dichotomous  branching,  and  the  conspicuous  rhombic  areas 
(areolae)  on  the  dorsal  surface ;  to  the  right  is  Anthoceros,  with  its  simple  thallus 
and  pod-like  sporogonia. — Goldberger. 


Fig.  94.  Cross-sections  of  thallus  of  Marchantia :  A,  section  from  thicker  part  of 
thallus,  where  supporting  tissue  (p)  is  abundant,  and  showing  lower  epidermis 
giving  rise  to  rhizoids  (h)  and  plates  (b),  also  chlorophyll  tissue  (chl)  organized 
into  chambers  by  partitions  (o)\  B,  section  near  margin  of  thallus  more  magnified, 
showing  lower  epidermis,  two  layers  of  supporting  tissue  {p)  with  reticulate  walls, 
a  single  chlorophyll  chamber  with  its  bounding  walls  (s)  and  containing  short, 
often  branching  filaments  whose  cells  contain  chloroplasts  (chl),  overarching 
upper  epidermis  (o)  pierced  by  a  large  chimney-like  air-pore  (sp).— After  Goebel. 


Fig.  95.  Section  through  cupule  of  Marchantia,  showing  wall  in  which  are  chloro- 
phyll-bearing air-chambers  with  air-pores,  and  gemmae  (a)  in  various  stages  of 
development.— Dodel- Port. 


Fig.  96.  Marchantia  polymorpha  :  the  lower  figure  represents  a  gametophyte  bear- 
ing a  mature  antheridial  branch  (d),  some  young  antheridial  branches,  and  also 
some  cupules  with  toothed  margins,  in  which  the  gemmae  may  be  seen ;  the 
upper  figure  represents  a  partial  section  through  the  antheridial  disk,  and  shows 
antheridia  within  the  antheridial  cavities  (a,  b,  c,  d,  «,/).— After  Knt. 


THE  GEEAT  GEOUPS  OF  BEYOPHYTES 


113 


cells  more  or  less  modified  for  conduction.  Above  these 
the  dorsal  region  is  organized  into  a  series  of  large  air  cham- 
bers, into  which  project  chlorophyll-containing  cells  in  the 


Fig.  97.  Marchantia  polymorpha,  a  common  liverwort :  1,  thallus,  with  rhizoids, 
bearing  a  mature  archegonial  branch  (/)  and  several  younger  ones  (a,  b,  c,  d,  «); 
2  and  3,  dorsal  and  ventral  views  of  archegonial  disk;  h  and  5,  young  sporophyte 
(sporogonium)  embryos;  o,  more  mature  sporogonium  still  within  enlarged  venter 
of  archegonium;  7,  mature  sporogonium  discharging  spores;  8,  three  spores  and 
an  elater. — After  Knt. 


form  of  short  branching  filaments.  Overarching  the  air 
chambers  is  the  dorsal  epidermis,  and  piercing  through  it 
into  each  air  chamber  is  a  conspicuous  air  pore  (Fig.  94,  B). 


114 


PLANT   STRUCT UKES 


The  air  chambers  are  outlined  on  the   surface  as   small 

rhombic  areas  (areola),  each  containing  a  single  air  pore. 

Peculiar  reproductive  bodies  are  also   developed  upon 

the  dorsal  surface  of  Marchantia  for  vegetative  multiplica- 


Fig.  98.  Marchantia  poiymorpha :  1,  partial  section  through  archegonial  disk,  show- 
ing archegonia  with  long  necke,  and  venters  containing  eggs;  9,  young  archego- 
nium  showing  axial  row;  10,  superficial  view  at  later  stage;  11.  mature  archego- 
nium,  with  axial  row  disorganized  and  leaving  an  open  passage  to  the  large  egg; 
1$,  cross-section  of  venter;  13,  cross-section  of  neck.— After  Knt. 


tion.  Little  cups  (cupules)  appear,  and  in  them  are  numer- 
ous short-stalked  bodies  (gemmce),  which  are  round  and 
flat  (biscuit-shaped)  and  many-celled  (Figs.  95,  96).     The 


THE  GREAT  GROUPS  OF  BRYOPHYTES       H5 

gemmae  fall  off  and  develop  new  thallus  bodies,  making 
rapid  multiplication  possible. 

Marchantia  also  possess  remarkably  prominent  gameto- 
phores,  or  "sexual  branches"  as  they  are  often  called. 
In  this  case  the  gametophores  are  differentiated,  one  bear- 
ing only  antheridia  (Fig.  96),  and  known  as  the  "anthe- 
ridial  branch,"  the  other  bearing  only  archegonia  (Figs.  97, 
98),  and  known  as  the  "archegonial  branch."  The  scal- 
loped antheridial  disk  and  the  star-shaped  archegonial  disk, 
each  borne  up  by  the  stalk-like  gametophore,  are  seen  in  the 
illustrations.  Not  only  are  the  gametophores  sexually  dif- 
ferentiated, but  as  only  one  appears  on  each  thallus,  the  thal- 
lus bodies  are  sexually  differentiated.  When  the  two  sex 
organs  appear  upon  different  individuals,  the  plant  is  said  to 
be  dioecious,  meaning  "  two  households  "  ;  when  they  both 
appear  upon  the  same  individual,  the  plant  is  monoecious, 
meaning  "  one  household."  Some  of  the  Bryophytes  are  mo- 
noecious, and  some  of  them  are  dioecious  (as  Marchantia). 

Another  distinguishing  mark  of  the  line  of  Marchantia 
forms  is  that  the  capsule-like  sporogonium  opens  irregu- 
larly to  discharge  its  spores  (Fig.  97,  7). 

70.  Jungermannia  forms. — This  is  the  greatest  line  of 
the  Liverworts,  the  forms  being  much  more  numerous 
than  in  the  other  lines.  They  grow  in  damp  places  ;  or  in 
drier  situations  on  rocks,  ground,  or  tree-trunks  ;  or  in  the 
tropics  also  on  the  leaves  of  forest  plants.  They  are  gen- 
erally delicate  plants,  and  resemble  small  Mosses,  many  of 
them  doubtless  being  commonly  mistaken  for  Mosses. 

This  resemblance  to  Mosses  suggests  one  of  the  chief 
features  of  the  line.  Beginning  with  a  simple  thallus,  as 
in  the  Marchantia  line,  the  structure  of  the  thallus  re- 
mains simple,  there  being  no  such  differentiation  of  tissues 
as  in  the  Marchantia  line  ;  but  the  form  of  the  thallus 
becomes  much  modified  (Figs.  99,  100).  Instead  of  a  flat 
thallus  with  even  outline,  the  body  is  organized  into  a  cen- 
tral stem-like  axis  bearing  two  rows  of  small,  often  crowded 


116 


PLANT   STRUCTUKES 


leaves.  There  are  really  three  rows  of  leaves,  but  the  third 
is  on  the  ventral  side  against  the  substratum,  and  is  often 
so  much  modified  as  not  to  look  like  the  other  leaves.  In 
consequence  of  this  the  Jungermannia  forms  are  usually 
called  "leafy  liverworts,"  to  distinguish  them  from  the 


Fig.  99.  Two  liverworts,  both  Jungermannia  forms:  to  the  left  is  Blasia,  which  re- 
tains the  thallus  form  but  has  lobed  margins;  to  the  right  is  Scapania,  with  dis- 
tinct leaves  and  sporogonia  (^4).— Goldbergeb. 


other  Liverworts,  which  are  "thallose."  They  are  also 
often  called  "scale  mosses,"  on  account  of  their  moss-like 
appearance  and  their  small  scale-like  leaves. 

The  line  may  be  distinguished,  therefore,  as  one  in 
which  the  differentiation  of  the  form  of  the  gametophyte 
is  emphasized.  Another  distinguishing  mark  is  that  the 
sporogonium  has  a  prominent  seta,  and  the  capsule  splits 
down  into  four  pieces  (valves)  when  opening  to  discharge 
the  spores  (Fig.  100,  0). 

71.  Anthoceros  forms. — This  line  contains  comparatively 
few  forms,  but  they  are  of  great  interest,  as  they  are  sup- 
posed to  represent  forms  which  have  given  rise   to  the 


THE  GREAT  GROUPS  OF  BRYOPHYTES       H7 


Pig.  100.  Species  of  Lfpidozia.  a  genua  of  leafy  liverworts,  showing  different  leaf 
forms,  and  in  A  and  ('  the  dehiscence  of  the  sporogonium  hy  four  valves.  In  C 
rhizoids  are  evident;  and  in  B,  D,  and  E  the  three  rows  of  leaves  are  seen,  the 
leaves  of  the  ventral  row  being  comparatively  small.— After  Englbk  and  Pkantl. 

Mosses,  and  possibly  to  the  Pteridophytes  also.  The 
thallus  is  very  simple,  being  differentiated  neither  in 
structure  nor  form,  as  in  the  two  other  lines ;   but  the 


118 


PLANT  STRUCTUKES 


special  development    has    been  in  connection  with    the 
sporogonium  (Figs.  93,  101). 

This  complex  sporogonium  (sporophyte)  has  a  large 
bulbous  foot  imbedded  in  the  simple  thallus,  while 
above  there  arises  a  long  pod-like  capsule.  The  com- 
plex walls  of  this  cap- 
sule contain  chlorophyll 
and  air  pores,  so  that 
the  sporogonium  is  or- 
ganized for  chlorophyll 
work.  If  it  could  send 
absorbing  roots  into  the 
soil,  this  sporophyte 
could  live  independent 
of  the  gametophyte.  In 
opening  to  discharge 
spores  the  pod-like  cap- 
sule splits  down  into 
two  valves. 

Another    peculiarity 

of  the  Antlioceros  forms 

is   in     connection    with 

m  jj       fa        the  antheridia  and  arch- 

fcj    vk*_    'Oh.       W        egonia.      These   organs, 

instead  of  growing  out 
free  from  the  body  of  the 
thallus,  as  in  other  Liv- 
erworts, are  imbedded  in 
it.     The  significance  of 
this    peculiarity  lies   in 
the  fact  that  it  is  a  char- 
acter which   belongs  to 
the  Pteridophytes. 
The  chief  direction  of  development  of  the  three  liv- 
erwort lines  may  be  summed  up  briefly  as  follows :   The 
Marchantia  line  has  differentiated   the   structure  of  the 


Fig.  101.  Anthoceros  gracilis :  A,  several 
gametophyte8,  on  which  sporogonia  have 
developed  ;  B,  an  enlarged  sporogonium, 
showing  its  elongated  character  and  de- 
hiscence by  two  valves  leaving  exposed 
the  slender  columella  on  the  surface  o' 
which  are  the  spores;  C,  D,  E,  F,  ela- 
ters  of  various  forms  ;  G,  spores. — After 
Schipfner. 


THE  GREAT  GEOUPS  OF  BRYOPHYTES       H9 

gametophyte  ;  the  Jungermannia  line  has  differentiated 
the  form  of  the  gametophyte ;  the  Anthoceros  line  has 
differentiated  the  structure  of  the  sporophyte.  It  should 
be  remembered  that  other  characters  also  serve  to  distin- 
guish the  lines  from  one  another. 

Musci  (Mosses) 

72.  General  character. — Mosses  are  highly  specialized 
plants,  probably  derived  from  Liverworts,  the  numerous 
forms  being  adapted  to  all  conditions,  from  submerged  to 
very  dry,  being  most  abundantly  displayed  in  temperate 
and  arctic  regions.  Many  of  them  may  be  dried  out  com- 
pletely and  then  revived  in  the  presence  of  moisture,  as  is 
true  of  many  Lichens  and  Liverworts,  with  which  forms 
Mosses  are  very  commonly  associated. 

They  also  have  great  power  of  vegetative  multiplica- 
tion, new  leafy  shoots  putting  out  from  old  ones  and  from 
the  protonema  indefinitely,  thus  forming  thick  carpets  and 
masses.  Bog  mosses  often  completely  fill  up  bogs  or  small 
ponds  and  lakes  with  a  dense  growth,  which  dies  below 
and  continues  to  grow  above  as  long  as  the  conditions  are 
favorable.  These  quaking  bogs  or  "mosses,"  as  they  are 
sometimes  called,  furnish  very  treacherous  footing  unless 
rendered  firmer  by  other  plants.  In  these  moss-filled  bog? 
the  water  shuts  off  the  lower  strata  of  moss  from  complete 
disorganization,  and  they  become  modified  into  a  coaly  sub« 
stance  called  peat,  which  may  accumulate  to  considerable 
thickness  by  the  continued  upward  growth  of  the  mass  of 
moss. 

The  gametophyte  body  is  differentiated  into  two  very 
distinct  regions :  (1)  the  prostrate  dorsiventral  thallus, 
which  is  called  protonema  in  this  group,  and  which  may  be 
either  a  broad  flat  thallus  (Fig.  104)  or  a  set  of  branching 
filaments  (Figs.  81,  102) ;  (2)  the  erect  leafy  branch  or 
gametophore  (Fig.  82).    This  erect  branch  is  said  to  be 


120 


PLANT   STRUCTURES 


radial,  in  contrast  with  the  dorsiventral  thallus,  referring 
to  the  fact  that  it  is  exposed  to  similar  conditions  all 
around,  and  its  organs  are  arranged  about  a  central  axis 
like  the  parts  of  a  radiate  animal.     This  position  is  much 

more  favorable  for  the 
chlorophyll  work  than 
the  dorsiventral  posi- 
tion, as  the  special 
chlorophyll  organs 
(leaves)  can  be  spread 
out  to  the  light  freely 
in  all  directions. 

It  should  be  re- 
marked that  the  gam- 
etophyte  in  all  groups 
of  plants  is  a  thallus, 
doing  its  chlorophyll 
work,  when  it  does 
any,  in  a  dorsiventral 
position  ;  the  only  ex- 
ception being  the  ra- 
dial leafy  branch  that 
arises  from  the  thal- 
lus of  Mosses.  From 
Mosses  onward  the 
gametophyte  becomes 
less  conspicuous,  so 
that  the  prominent 
leafy  plants  of  the 
higher  groups  hold  no 
relation  to  the  little  erect  leafy  branch  of  the  Mosses, 
which  is  put  out  by  the  gametophyte,  and  which  is  the 
best  the  gametophyte  ever  does  toward  getting  into  a  bet- 
ter position  for  chlorophyll  work. 

The  leafy  branch  of  the  Mosses  usually  becomes  inde- 
pendent of  the  thallus  by  putting  out  rhizoids  at  its  base 


Fig.  102.  A  moss  Uiryum),  showing  base  of  a 
leafy  branch  (gamctophore)  attached  to  the 
protonema.  and  having  sent  out  rhizoids.  On 
the  protonemal  filament  to  the  right  and  be- 
low is  the  young  bud  of  another  leafy  branch. 

— MfJLLER. 


THE  GKEAT  GROUPS  OF  BKYOPHYTES 


121 


(Fig.  102),  the  thallus  part  dying.  Sometimes,  however, 
the  filamentous  protonema  is  very  persistent,  and  gives  rise 
to  a  perennial  succession  of  leafy  branches. 

A  ,J» 


Fig.  103.  Tip  of  leafy  branch  of  a  moss  (Funaria),  bearing  a  cluster  of  sex  organs, 
showing  an  old  antheridium  (A),  a  younger  one  (_B),  some  of  the  curious  associated 
hairs  (p),  and  leaf  sections  (/).— After  Campbell. 


At  the  summit  of  the  leafy  gametophore,  either  upon 
the  main  axis  or  upon  a  lateral  branch,  the  antheridia  and 
archegonia  are  borne  (Figs.  83,  103).  Often  the  leaves  at 
the  summit  become  modified  in  form  and  arranged  to  form 


122 


PLANT  STKUCTUKES 


a  rosette,  in  the  center  of  which  are  the  sex  organs.  This 
rosette  is  often  called  the  "moss  flower,"  but  it  holds  no 
relation  to  the  flower  of  Seed-plants,  and  the  phrase  should 
not  be  used.  A  rosette  may  contain  but  one  kind  of  sex 
organ  (Figs.  83,  103),  or  it  may  contain  both  kinds,  for 
Mosses  are  both  dioecious  and  monoecious.  The  two  prin- 
cipal groups  are  as  follows  : 

73.  Sphagnum  forms. — These  are  large  and  pallid  bog 
mosses,  found  abundantly  in  marshy  ground,  especially  of 
temperate  and  arctic  regions,  and  are  conspicuous  peat- 
formers  (Fig.  105,  A).     The  leaves  and  gametophore  axis 

are  of  peculiar  struc- 
ture to  enable  them 
to  suck  up  and  hold 
a  large  amount  of  wa- 
ter. This  abundant 
water  -  storage  tissue 
and  the  comparative- 
ly poor  display  of 
chlorophyll  -  contain- 
ing cells  gives  the 
peculiar  pallid  ap- 
pearance. 

They  resemble  the 
Liverworts  in  the 
broad  thallus  body 
of  the  gametophyte, 
from  which  the  large 
leafy  gametophore 
arises  (Fig.  104). 
They  also  resemble 
Anthoceros  forms  in  the  sporogonium,  the  archesporium 
being  a  dome-shaped  mass  (Fig.  105,  C).  On  the  other 
hand,  they  resemble  the  true  Mosses,  not  only  in  the  leafy 
gametophore,  but  also  in  the  fact  that  the  capsule  opens 
at  the  apex  by  a  circular  lid,  called  the  operculum  (Fig. 


Fig.  104.  Thallus  body  of  gametophyte  of  Sphag- 
num, giving  rise  to  rhizoids  (r)  and  buds  (k) 
which  develop  into  the  large  leafy  branches 
(gametophores).—  After  Campbell. 


THE  GREAT  GROUPS  OF  BRYOPHYTES 


123 


105,  D),  which  means  a  "cover"  or  "lid."  This  may 
serve  to  illustrate  what  is  called  an  "intermediate"  or 
"transition"  type,  Sphagnum  showing  characters  which 
ally  it  to^  Anthoceros  forms  on  the  one  side,  and  to  true 
Mosses  on  the  other. 

A  peculiar  feature  of  the  sporogonium  is  that  it  has  no 
long  stalk-like  seta,  as  have  the  true  Mosses,  although  it 
appears  to  have  one.    This  false  appearance  arises  from  the 


Fig.  105.  Sphagnum  :  A,  a  leafy  branch  (gametophore)  bearing  four  mature  sporo- 
gonia;  B,  archegonium  in  whose  venter  a  young  embryo  sporophyte  (em)  is  de- 
veloping; C,  section  of  a  young  sporogonium  (sporophyte),  showing  the  bulbous 
foot  (spf)  imbedded  in  the  apex  of  the  pseudopodium  (ps),  the  capsule  (k),  the 
columella  (co)  capped  by  the  dome-shaped  archesporium  (spo),  a  portion  of  the 
calyptra  (ca),  and  the  old  archegonium  neck  (ah);  D,  branch  bearing  mature 
sporogonium  and  showing  pseudopodium  (ps),  capsule  (k),  and  operculum  (d);  E, 
antberidium  discharging  sperms;  F,  a  single  sperm,  showing  coiled  body  and  two 
cilia. — After  Schimper. 


fact  that  the  axis  of  the  gametophore  is  prolonged  above 
its  leafy  portion,  the  prolongation  resembling  the  seta  of 
an  ordinary  moss   (Fig.  105,  D).     This  prolongation   is 


124 


PLANT  STRUCTURES 


called  a  pseudopodium,  or  "  false  stalk,"  and  in  the  top  of 
it  is  imbedded  the  foot  of  the  sporogonium  carrying  the 
globular  capsule  (Fig.  105,  C). 

74.  True  Mosses. — This  immense  and  most  highly  organ- 
ized Bryophyte  group  contains  the  great  majority  of  the 
Mosses,  which  are  sometimes  called  the  Bryum  forms,  to 
distinguish  them  from  the  Sphagnum  forms.      They  are 


Fig.  106.  Different  stages  in  the  development  of  the  leafy  gametophore  from  the  pro- 
tonema  of  a  common  moss  (Fimaria):  A,  the  first  few  cells  and  a  rhizoid  (r);  B, 
C,  later  stages,  showing  apical  cell  (;)  and  young  leaves  (2);  D,  later  stage  much 
less  magnified,  showing  protonemal  filaments  and  the  young  gametophore  (gam) 
— After  Camtbell. 


the  representative  Bryophytes,  the  only  group  vying  with 
them  being  the  leafy  Liverworts,  or  Jungermannia  forms. 
They  grow  in  all  conditions  of  moisture,  from  actual  sub- 
mergence in  water  to  dry  rocks,  and  they  also  form  exten- 
sive peat  deposits  in  bogs. 

The  thallus  body  of  the  gametophyte  is  made  up  of 
branching  filaments  (Figs.  81,  102),  those  exposed  to  the 


THE  GREAT  GROUPS  OF  BRYOPHYTES 


125 


light  containing  chlorophyll,  and  those  in  the  substratum 
being  colorless  and  acting  as  rhizoids.  The  leafy  gameto- 
phores  are  often  highly  organized  (Figs.  102,  106),  the 
leaves  and  stems  showing  a  certain  amount  of  differentia- 
tion of  tissues. 

It  is  the  sporophyte,  however,  which  shows  the  great- 
est amount  of  serialization  (Fig.  107).     The  sporogonium 


Fig.  107.  A  common  moss  (Funaria):  in  the  center  is  the  leafy  shoot  (gametophore), 
with  rhizoids,  several  leaves,  and  a  sporogonium  (sporophyte),  with  a  long  seta, 
capsule,  and  at  its  tip  the  calyptra  {cal);  to  the  right  a  capsule  with  calyptra  re- 
moved, showing  the  operculum  (o);  to  the  left  a  young  sporogonium  pushing  up 
the  calyptra  from  the  leafy  shoot.— After  Campbell. 


has  a  foot  and  a  long  slender  seta,  but  the  capsule  is  espe- 
cially complex.  The  archesporium  is  reduced  to  a  small 
hollow  cylinder  (Fig.  88),  the  capsule  wall  is  most  elabo- 
rately constructed,  and  the  columella  runs  through  the 
27 


Fig.  108.  Longitudinal  section  of  moss  capsule 
(Funaria),  showing  its  complex  character: 
d,  operculum;  p,  peristome:  c,  c',  columel- 
la; «,  sporogenous  tissue ;  outside  of  s  the 
complex  wall  consisting  of  layers  of  cells 
and  large  open  spaces  (h)  traversed  by 
strands  of  tissue.— After  Goebel. 


Pig.  110.  Sporogonia  of  Gtimmia,  from  all  of 
which  the  operculum  has  fallen,  displaying 
the  peristome  teeth :  A,  position  of  the  teeth 
when  dry;  B,  position  when  moist. — After 
Kekner. 


Fig.  109.  Partial  longitudinal 
section  through  a  moss  cap- 
sule :  A,  younger  capsule, 
showing  wall  cells  (a),  cells 
of  columella  (i),  and  sporog- 
enous cells  (su) ;  B,  some- 
what older  capsule,  a  and  i 
same  as  before,  and  sm  the 
spore  mother  cells.  —  After 
Goebel. 


THE  GREAT  GROUPS  CF  BRYOPHYTES       ^27 

center  of  the  capsule  to  the  lid-like  operculum  (Figs.  108, 
109).  When  the  operculum  falls  off  the  capsule  is  left  like 
an  urn  full  of  spores,  and  at  the  mouth  of  the  urn  there  is 
usually  displayed  a  set  of  slender,  often  very  beautiful  teeth 
(Fig.  110),  converging  from  the  circumference  toward  the 
center,  and  called  the  peristome,  meaning  "  about  the 
mouth."  These  teeth  are  hygroscopic,  and  by  bending 
inward  and  outward  help  to  discharge  the  spores. 


CHAPTEK  IX 

PTERIDOPHYTES  (FERN  PLANTS) 

75.  Summary  from  Bryophytes. — In  introducing  the  Bryo- 
phytes  a  summary  from  the  Thallophytes  was  given  (see  § 
60),  indicating  certain  important  things  which  that  group 
has  contributed  to  the  evolution  of  the  plant  kingdom. 
In  introducing  the  Pteridophytes  it  is  well  to  notice  certain 
important  additions  made  by  the  Bryophytes. 

(1)  Alternation  of  generations. — The  great  fact  of  alter- 
nating sexual  (gametophyte)  and  sexless  (sporophyte)  gen- 
erations is  first  clearly  expressed  by  the  Bryophytes,  although 
its  beginnings  are  to  be  found  among  the  Thallophytes. 
Each  generation  produces  one  kind  of  spore,  from  which  is 
developed  the  other  generation. 

(2)  Gametophyte  the  chlorophyll  generation. — On  account 
of  this  fact  the  food  is  chiefly  manufactured  by  the  gameto- 
phyte, which  is  therefore  the  more  conspicuous  generation. 
When  a  moss  or  a  liverwort  is  spoken  of,  therefore,  the 
gametophyte  is  usually  referred  to. 

(3)  Gametophyte  and  sporophyte  not  independent. — The 
sporophyte  is  mainly  dependent  upon  the  gametophyte  for 
its  nutrition,  and  remains  attached  to  it,  being  commonly 
called  the  sporogonium,  and  its  only  function  is  to  produce 
spores. 

(4)  Differentiation  of  thalhis  into  stem  and  leaves. — 
This  appears  incompletely  in  the  leafy  Liverworts  (Junger- 
mannia  forms)  and  much  more  clearly  in  the  erect  and 
radial  leafy  branch  (gametophore)  of  the  Mosses. 

128 


PTERIDOPHYTES  129 

(5)  Many-celled  sex  o?'gans. — The  antheridia  and  the 
flask-shaped  archegonia  are  very  characteristic  of  Bryo- 
phytes  as  contrasted  with  Thallophytes. 

76.  General  characters  of  Pteridophytes. — The  name  means 
"  fern  plants/"'  and  the  Ferns  are  the  most  numerous  and  the 
most  representative  forms  of  the  group.  Associated  with 
them,  however,  are  the  Horsetails  (Scouring  rushes)  and 
the  Club-mosses.  By  many  the  Pteridophytes  are  thought 
to  have  been  derived  from  such  Liverworts  as  the  Antho- 
ceros  forms,  while  some  think  that  they  may  possibly  have 
been  derived  directly  from  the  Green  Alga?.  Whatever 
their  origin,  they  are  very  distinct  from  Bryophytes. 

One  of  the  very  important  facts  is  the  appearance  of 
the  vascular  system,  which  means  a  "system  of  vessels," 
organized  for  conducting  material  through  the  plant  body. 
The  appearance  of  this  system  marks  some  such  epoch  in 
the  evolution  of  plants  as  is  marked  in  animals  by  the 
appearance  of  the-  "backbone."  As  animals  are  often 
grouped  as  "vertebrates"  and  "invertebrates,"  plants  are 
often  grouped  as  "vascular  plants"  and  "non-vascular 
plants,"  the  former  being  the  Pteridophytes  and  Spermato- 
phytes,  the  latter  being  the  Thallophytes  and  Bryophytes. 
Pteridophytes  are  of  great  interest,  therefore,  as  being  the 
first  vascular  plants. 

77.  Alternation  of  generations. — This  alternation  con- 
tinues in  the  Pteridophytes,  but  is  even  more  distinct  than 
in  the  Bryophytes,  the  gametophyte  and  sporophyte  be- 
coming independent  of  one  another.  An  outline  of  the  life 
history  of  an  ordinary  fern  will  illustrate  this  fact,  and  will 
serve  also  to  point  out  the  prominent  structures.  Upon  the 
lower  surface  of  the  leaves  of  an  ordinary  fern  dark  spots 
or  lines  are  often  seen.  These  are  found  to  yield  spores, 
with  which  the  life  history  may  be  begun. 

When  such  a  spore  germinates  it  gives  rise  to  a  small, 
green,  heart-shaped  thallus,  resembling  a  delicate  and  sim- 
ple liverwort  (Fig.  Ill,  A).     Upon  this  thallus  antheridia 


130 


PLANT   STRUCT UEES 


and  archegonia  appear,  so  that  it  is  evidently  a  gameto* 
phyte.  This  gametophyte  escapes  ordinary  attention,  as  it 
is  usually  very  small,  and  lies  prostrate  upon  the  substra- 
tum. It  has  received  the  name  prothallium  or  prothattus, 
so  that  when  the  term  prothallium  is  used  the  gametophyte 
of  Pteridophytes  is  generally  referred  to  ;  just  as  when  the 
term  sporogonium  is  used  the  sporophyte  of  the  Bryophytes 
is  referred  to.  Within  an  archegonium  borne  upon  this  little 
prothallium  an  oospore  is  formed.     When  the  oospore  ger- 


Fig.  111.  Prothallium  of  a  common  fern  (A$pidium):  A,  ventral  surface,  showing 
rhizoids  (rA),  antheridia  {an),  and  archegonia  (.«/•) ;  B,  ventral  surface  of  an  older 
gametophyte,  showing  rhizoids  (rh)  and  young  sporophyte  with  root  (w)  and  leaf 
(6).— After  Schenck. 

minates  it  develops  the  large  leafy  plant  ordinarily  spoken 
of  as  "  the  fern,"  with  its  subterranean  stem,  from  which 
roots  descend,  and  from  which  large  branching  leaves  rise 
above  the  surface  of  the  ground  (Fig.  Ill,  B).  It  is  in 
this  complex  body  that  the  vascular  system  appears.  No 
sex  organs  are  developed  upon  it,  but  the  leaves  bear  numer- 
ous sporangia  full  of  asexual  spores.  This  complex  vascular 
plart,  therefore,  is  a  sporophyte,  and  corresponds  in  this 
life  history  to  the  sporogonium  of  the  Bryophytes.     This 


PTEEIDOPHYTES  131 

completes  the  life  cycle,  as  the  asexual  spores  develop  the 
prothallium  again. 

In  contrasting  this  life  history  with  that  of  Bryophytes 
several  important  differences  are  discovered.  The  most 
striking  one  is  that  the  sporophyte  has  become  a  large, 
leafy,  vascular,  and  independent  structure,  not  at  all  re- 
sembling its  representative  (the  sporogonium)  among  the 
Bryophytes. 

Also  the  gametophyte  is  much  less  prominent  than  the 
gametophytes  of  the  larger  Liverworts  and  Mosses.  If 
Ferns  have  been  derived  from  the  Liverworts,  therefore,  it 
is  probable  that  they  came  from  those  with  very  simple 
bodies  rather  than  from  those  in  which  the  gametophyte 
had  become  large  and  complex.  The  conspicuous  leafy 
branch  of  the  Mosses,  commonly  called  "the  moss  plant," 
corresponds  to  nothing  in  the  Pteridophytes,  the  prothal- 
lium representing  only  the  protonema  part  of  the  gameto- 
phyte of  the  true  Mosses. 

The  small  size  of  the  gametophyte  seems  to  be  associ- 
ated with  the  fact  that  the  chlorophyll  work  has  been 
transferred  to  the  sporophyte,  which  hereafter  remains  the 
conspicuous  generation.  The  "  fern  plant "  of  ordinary 
observation,  therefore,  is  the  sporophyte ;  while  the  "  moss 
plant "  is  a  leafy  branch  of  the  gametophyte.. 

Another  important  contrast  indicated  is  that  in  Bryo- 
phytes the  sporophyte  is  dependent  upon  the  gametophyte 
for  its  nutrition,  remaining  attached  to  it ;  while  in  most 
of  the  Pteridophytes  both  generations  are  independent 
green  plants,  the  leafy  sporophyte  remaining  attached  to 
the  small  gametophyte  only  while  beginning  its  growth 
(Fig.  Ill,  B). 

Among  the  Ferns  some  interesting  exceptions  to  this 
method  of  alternation  have  been  observed.  Under  certain 
conditions  a  leafy  sporophyte  may  sprout  directly  from  the 
prothallium  (gametophyte)  instead  of  from  an  oospore. 
This  is  called  apogamy,  meaning  "  without  the  sexual  act/' 


132 


PLANT   STKUCTUKES 


Under  certain  other  conditions  prothallia  are  observed  to 
sprout  directly  from  the  leafy  sporophyte  instead  of  from 
a  spore.  This  is  called  apospory,  meaning  "without  a 
spore." 

78.  The  gametophyte. — The  prothallium,  like  a  simple 
liverwort,  is  a  dorsi ventral  body,  and  puts  out  numerous 


Fig.  112.  Stag-horn  fern  (Plalycerium  grande),  an  epiphytic  tropical  form,  showing 
the  two  forms  of  leaves:  a  and  b,  young  sterile  leaves;  c,  leaves  bearing  spo- 
rangia ;  d,  an  old  sterile  leaf. — Caldwell. 


rhizoids  from  its  ventral  surface  (Fig.  111).  It  is  so  thin 
that  all  the  cells  contain  chlorophyll,  and  it  is  usually  short- 
lived.    In  rare  cases  it  becomes  quite  large  and  permanent, 


Fig.  113.  Archegoniuru  of  Pteris  at  the  time  of  fertilization,  showing  tissue  of  gam- 
etophyte  (A),  the  cells  forming  the  neck  (B),  the  passageway  formed  by  the  dis- 
organization of  the  canal  cells  (C),  and  the  egg  (Z»  lying  exposed  in  the  venter. 
— Caldwell. 


Fig.  114.  Antheridium  of  Pteris  (B),  showing  wall  cells  (</),  opening  for  escape  of 
sperm  mother  cells  (e),  escaped  mother  cells  (c),  sperms  free  from  mother  cells  (6), 
showing  spiral  and  multiciliate  character. — Caldwell. 


134 


PLANT   STKUCTUKES 


being  a  conspicuous  object  in  connection  with  the  sporo- 
phyte. 

At  the  bottom  of  the  conspicuous  notch  in  the  prothal- 
lium  is  the  growing  point, 
representing  the  apex  of  the 
plant.     This  notch  is  always 
a  conspicuous  feature. 

The  antheridia  and  arch- 
egonia  are  usually  developed 
on  the  under  surface  of  the 
prothallium  (Fig.  Ill,  A), 
and  differ  from  those  of  all 
Bryophytes,  except  the  An- 
tlwceros  forms,  in  being  sunk 
in  the  tissue  of  the  prothal- 
lium and  opening  on  the  sur- 


Pig.  115.  Development  of  gametophyte 
of  Pteris:  the  figure  to  the  left  shows 
the  old  spore  ( B),  the  rhizoid  ( C),  and 
the  t'nallus  (A);  that  to  the  right  is 
older,  showing  the  same  parts,  and 
also  the  apical  cell  (B). — Caldwell. 


Fig.  116.  Young  gametophyte  of  Pteris, 
showing  old  spore  wall  ( /?),  rhizoids 
(£')>  apical  cell  (D),  a  young  anther- 
idium  (E),  and  an  older  one  in  which 
sperms  have  organized  (F). — Cald- 
well. 


PTEKIDOPHYTES 


135 


face,  more  or  less  of  the  neck  of  the  archegonium  projecting 
(Fig.  113).  The  eggs  are  not  different  from  those  formed 
within  the  archegonia  of  Bryophytes,  but  the  sperms  are 
very  different.  The  Bryophyte  sperm  has  a  small  body  and 
two  long  cilia,  while  the  Pteridophyte  sperm  has  a  long 
spirally  coiled  body,  blunt  behind  and  tapering  to  a  point  in 
front,  where  numerous  cilia  are  developed  (Fig.  114).  It 
is,  therefore,  a  large,  spirally-coiled,  multiciliate  sperm,  and 
is  quite  characteristic  of  all  Pteridophytes  excepting  the 
Club-mosses.  It  is  evident  that  a  certain  amount  of  water 
is  necessary  for  fertilization — in  fact,  it  is  needed  not  only 


Fig.  117.  Sections  of  portions  of  the  gametophyte  of  Pteris,  showing  development 
of  archegonium:  A,  young  stage,  showing  cells  which  develop  the  neck  (a),  and 
the  cell  from  which  the  egg  cell  and  canal  cells  develop  (6);  B,  an  older  stage, 
showing  neck  cells  (a),  neck  canal  cell  (6),  and  cell  from  which  is  derived  the  egg 
cell,  and  the  ventral  canal  cell  (c);  C,  a  still  older  stage,  showing  increased  num- 
ber of  neck  cells  (a),  two  neck  canal  cells  (6),  the  ventral  canal  cell  (c),  and  the 
cell  in  which  the  egg  is  organized  (d).— Caldwell. 


by  the  swimming  sperm,  but  also  to  cause  the  opening  of 
the  antheridium  and  of  the  archegonium  neck.  There 
seems  to  be  a  relation  between  the  necessity  of  water  for 
fertilization  and  a  prostrate,  easily  moistened  gametophyte. 
Prothallia  are  either  monoecious  or  dioecious  (see  §  69). 
When  the  prothallia  are  developing  (Fig.  115)  the  anther- 


Fig.  118.  A  fern  (Aspidivm),  showing  three  large  branching  leaves  coming  from  a 
horizontal  subterranean  stem  (rootstock):  young  leaves  are  also  shown,  which 
show  circinate  vernation.  The  stem,  young  leaves,  and  petioles  of  the  large 
leaves  are  thickly  covered  with  protecting  hairs.  The  stem  gives  rise  to  numerous 
small  roots  from  its  lower  surface.  The  figure  marked  3  represents  the  under  sur- 
face of  a  portion  of  the  leaf,  showing  seven  sori  with  shield-like  indusia;  at  5  is 
represented  a  section  through  a  sorus.  showing  the  sporangia  attached  and  pro- 
tected by  the  indusium;  while  at  6"  is  represented  a  single  sporangium  opening 
and  discharging  its  spores,  the  heavy  annulus  extending  along  the  back  and  over 
the  top.— After  Wossidlo. 


TTERIDOrHYTES 


137 


idia  begin  to  appear  very  early  (Fig.  116),  and  later  the 
archegonia  (Fig.  117).  If  the  prothallium  is  poorly  nour- 
ished, only  antheridia  appear ;  it  needs  to  be  well  developed 
and  nourished  to  develop  archegonia.  There  seems  to  be 
a  very  definite  relation,  therefore,  between  nutrition  and 
the  development  of  the  two  sex  organs,  a  fact  which  must 
be  remembered  in  connection  with  the  development  of 
heterospory. 

79.  The  sporophyte. — This  complex  body  is  differentiated 
into  root,  stem,  and  leaf,  and  is  more  highly  organized 
than  any  plant  body  heretofore  mentioned  (Fig.  118).  The 
development  of  this  body  and  its  three  great  working  regions 
must  be  considered  separately. 

(1)  Development  of  embryo. — The  oospore,  from  which 
the  sporophyte  develops,  rests  in  the  venter  of  the  arche- 
gonium,  which  at  this  stage  resembles  a  depression  in  the 


Pig.  119.  Embryos  of  a  common  fern  {Pterin):  A,  young  embryo,  showing  direction 
of  basal  wall  (/),  and  of  second  walls  (II),  which  organize  quadrants,  each  of 
which  subsequently  develops  into  foot  (/),  root  (w),  leaf  (b),  and  stem  (*■);  B,  an 
older  embryo,  in  which  the  four  regions  (lettered  as  in  A)  have  developed  further, 
also  showing  venter  of  archegonium  (aw),  and  some  tissue  of  the  prothallium  (pr). 
—A  after  Kienitz-CJerlopp;  B  after  IIofmeister. 


lower  surface  of  the  prothallium  (Fig.  119,  B).  It  germi- 
nates at  once,  as  in  Bryophytes,  not  being  a  resting  spore 
as  in  Green  Algae.    The  resting  stage,  as  in  the  Bryophytes, 


J38  PLANT   STRUCTURES 

is  in  connection  with  the  asexual  spores,  which  may  be 
kept  for  a  long  time  and  then  germinated. 

The  first  step  in  germination  is  for  the  oospore  to  di- 
vide into  two  cells,  forming  a  two-celled  embryo.  In  the 
ordinary  Ferns  this  first  dividing  wall  is  at  right  angles  to 
the  surface  of  the  prothallium,  and  is  called  the  basal  wall 
(Fig.  119,  A).  One  of  the  two  cells,  therefore,  is  anterior 
(toward  the  notch  of  the  prothallium),  and  the  other  is 
posterior. 

The  two  cells  next  divide  by  forming  walls  at  right 
angles  to  the  basal  wall,  and  a  four-celled  embryo  is  the 
result.  This  is  called  the  "quadrant  stage"  of  the  em- 
bryo, as  each  one  of  the  four  cells  is  like  the  quadrant  of  a 
sphere. 

With  the  appearance  of  the  quadrant,  four  body  regions 
are  organized,  each  cell  by  its  subsequent  divisions  giving 
rise  to  a  distinct  working  region  (Fig.  119,  A).  Two  of  the 
cells  are  inner  (away  from  the  substratum)  ;  also  one  of  the 
inner  and  one  of  the  outer  (toward  the  substratum)  cells 
are  anterior  ;  while  the  two  other  inner  and  outer  cells  are 
posterior.  The  anterior  outer  cell  develops  the  first  leaf  of 
the  embryo,  generally  called  the  cotyledon  (Fig.  119,  b) ;  the 
anterior  inner  cell  develops  the  stem  (Fig.  119,  s)  ;  the  pos- 
terior outer  cell  develops  the  first  (primary)  root  (Fig. 
119,  w) ;  the  posterior  inner  cell  develops  a  special  organ 
for  the  use  of  the  embryo,  called  the  foot  (Fig.  119,  /). 
The  foot  remains  in  close  contact  with  the  prothallium  and 
absorbs  nourishment  from  it  for  the  young  embryo.  When 
the  young  sporophyte  has  developed  enough  to  become  in- 
dependent the  foot  disappears.  It  is  therefore  spoken  of 
as  a  temporary  organ  of  the  embryo.  It  is  necessary  for  the 
leaf  to  emerge  from  beneath  the  prothallium,  and  it  may 
be  seen  usually  curving  upward  through  the  notch.  The 
other  parts  remain  subterranean. 

(2)  The  root. — The  primary  root  organized  by  one  of 
the  quadrants  of  the  embryo  is  a  temporary  affair  (Figs. 


PTERIDOPHYTES  139 

111,  119),  as  it  is  in  an  unfavorable  position  in  reference  to 
the  dorsiventral  stem,  which  puts  out  a  series  of  more  favor- 
ably placed  secondary  roots  into  the  soil  (Fig.  118).  The 
mature  leafy  sporophyte,  therefore,  has  neither  foot  nor 
primary  root,  the  product  of  two  of  the  quadrants  of  the 
embryo  having  disappeared. 

The  secondary  roots  put  out  by  the  stem  are  small,  and 
do  not  organize  an  extensive  system,  but  they  are  interest- 
ing as  representing  the  first  appearance  of  true  roots,  which 
therefore  come  in  with  the  vascular  system.  In  the  lower 
groups  the  root  function  of  absorption  is  not  assumed  by 
any  special  organ,  unless  rhizoids  sometimes  absorb ;  but 
true  roots  are  complex  in  structure  and  contain  vessels. 

(3)  The  stem. — In  most  of  the  Ferns  the  stem  is  sub- 
terranean and  dorsiventral  (Fig.  118),  but  in  the  "tree 
ferns  "  of  the  tropics  it  forms  an  erect,  aerial  shaft  bearing 
a  crown  of  leaves  (Fig.  120).  In  the  other  groups  of  Pteri- 
dophytes  there  are  also  aerial  stems,  both  erect  and  pros- 
trate. The  stem  is  complex  in  structure,  the  cells  being 
organized  into  different  "  tissue  systems,"  prominent  among 
which  is  the  vascular  system.  These  tissue  systems  of  vas- 
cular plants  are  described  in  Chapter  XV. 

The  appearance  of  the  vascular  system  in  connection 
with  the  leafy  sporophyte  is  worthy  of  note.  The  leaves 
are  special  organs  for  chlorophyll  work,  and  must  receive 
the  raw  material  from  air  and  soil  or  water.  The  leaves 
of  the  moss  gametophyte  are  very  small  and  simple  affairs, 
and  can  be  supplied  with  material  by  using  very  little  ap- 
paratus. In  the  leafy  sporophyte,  however,  the  leaves  are 
very  prominent  structures,  capable  of  doing  a  great  deal 
of  work.  To  such  working  structures  material  must  be 
brought  rapidly  in  quantity,  and  manufactured  food  ma- 
terial must  be  carried  away,  and  therefore  a  special  con- 
ducting apparatus  is  needed.  This  is  supplied  by  the  vas- 
cular system.  These  vessels  extend  continuously  from  root- 
tips,  through  the  stem,  and  out  into  the  leaves,  where  they 


Fie.  120.  A  group  of  tropical  plants.  To  the  left  of  the  center  is  a  tree  fern,  with  its 
slender  columnar  stem  and  crown  of  large  leaves.  The  large-leaved  plants  to  the 
right  are  bananas  (Monocotyledons).— From  "  Plant  Relations." 


PTEKIDOPHYTES 


141 


are  spoken  of  as  "leaf  veins."  Large  working  leaves  and 
a  vascular  system,  therefore,  belong  together  and  appear 
together;  and  the  vascular  plants  are  also  the  plants  with 
leafy  sporophytes. 

(4)  The  leaf. — Leaves  are  devices  for  spreading  out 
green  tissue  to  the  light,  and  in  the  Ferns  they  are  usually 
large.  There  is  a  stalk-like  portion  (petiole)  which  rises 
from  the  subterranean  stem,  and  a  broad  expanded  portion 
(blade)  exposed  to  the  light  and  air  (Fig.  118).  In  Ferns 
the  blade  is  usually  much  branched,  being  cut  up  into 
segments  of  various  sizes  and  forms. 

The  essential  structure  consists  of  an  expansion  of 
green  tissue  (mesophyll),  through  which  strands  of  the 
vascular  system  (veins)  branch,  forming  a  supporting 
framework,  and  over  all  a  compact  layer  of  protecting 
cells  (epidermis).  A  surface 
view  of  the  epidermis  shows 
that  it  is  pierced  by  numer- 
ous peculiar  pores,  called 
stomata,  meaning  "  mouths." 
The  surface  view  of  a  stoma 
shows  two  crescentic  cells 
(guard  cells)  in  contact  at 
the  ends  and  leaving  be- 
tween them  a  lens-shaped 
opening  (Fig.  121). 

A  cross-section  through 
a  leaf  gives  a  good  view  of 
the  three  regions  (Fig.  122). 
Above  and  below  is  the  col- 
orless    epidermis,     pierced 
here  and  there  by  stomata ; 
between  the  epidermal  lay- 
ers  the  cells  of   the  mesophyll   are   packed;  and   among 
the  mesophyll  cells  there  may  be  seen  here  and  there  the 
cut  ends  of  the  veins.     The  leaf  is  usually  a  dorsiventral 
28 


Fig.  121.  Some  epidermal  cells  from 
leaf  of  Pteris,  showing  the  inter- 
locking walls  and  three  stomata,  the 
guard  cells  containing  chloroplasts. 
— Land. 


142 


PLANT   STEUCTUEES 


organ,  its  two  surfaces  being  differently  related  to  light. 
To  this  different  relation  the  mesophyll  cells  respond  in 
their  arrangement.  Those  in  contact  with  the  upper  epi- 
dermis become  elongated  and  set  endwise  close  together, 
forming  the  palisade  tissue;  those  below  are  loosely  ar- 


Fio.  122.  Cross-section  through  a  portion  of  the  leaf  of  Pteris,  showing  the  heavy- 
walled  epidermis  above  and  below,  two  stomata  in  the  lower  epidermis  (one  on 
each  side  of  the  center)  opening  into  intercellular  passages,  the  mesophyll  cells 
containing  chloroplasts,  the  upper  row  arranged  in  palisade  fashion,  the  other 
cells  loosely  arranged  (spongy  mesophyll)  and  leaving  large  intercellular  passages, 
and  in  the  center  a  section  of  a  veinlet  (vascular  bundle),  the  xylem  being  repre- 
sented by  the  central  group  of  heavy-walled  cells.— Land. 


ranged,  leaving  numerous  intercellular  spaces,  forming 
the  spongy  tissue.  These  spaces  form  a  system  of  inter- 
cellular passageways  among  the  working  mesophyll  cells, 
putting  them  into  communication  with  the  outer  air 
through  the  stomata.    The  freedom  of  this  communication 


PTERIDOPHYTES  143 

is  regulated  by  the  guard  cells  of  the  stomata,  which  come 
together  or  shrink  apart  as  occasion  requires,  thus  dimin- 
ishing or  enlarging  the  opening  between  them.  The  sto- 
mata have  well  been  called  "automatic  gateways"  to  the 
system  of  intercellular  passageways. 

One  of  the  peculiarities  of  ordinary  fern  leaves  is 
that  the  vein  system  branches  dichotomously,  the  forking 
veins  being  very  conspicuous  (Figs.  123-126).  Another 
fern  habit  is  that  the  leaves  in  expanding  seem  to  unroll 
from  the  base,  as  though  they  had  been  rolled  from  the 
apex  downward,  the  apex  being  in  the  center  of  the  roll 
(Fig.  118).  This  habit  is  spoken  of  as  circinate,  from  a 
word  meaning  "circle"  or  "coil,"  and  circinate  leaves 
when  unrolling  have  a  crozier-like  tip.  The  arrangement 
of  leaves  in  bud  is  called  vernation  ("spring  condition"), 
and  therefore  the  Ferns  are  said  to  have  circinate  verna- 
tion. The  combination  of  dichotomous  venation  and  cir- 
cinate vernation  is  very  characteristic  of  Ferns. 

80.  Sporangia. — Among  Thallophytes  sporangia  are  usu- 
ally simple,  mostly  consisting  of  a  single  mother  cell ;  among 
Bryophytes  simple  sporangia  do  not  exist,  and  in  connec- 
tion with  the  usually  complex  capsule  of  the  sporogonium 
the  name  is  dropped ;  but  among  Pteridophytes  distinct 
sporangia  again  appear.  They  are  not  simple  mother  cells, 
but  many-celled  bodies.  Their  structure  varies  in  different 
groups  of  Pteridophytes,  but  those  of  ordinary  Ferns  may 
be  taken  as  an  illustration. 

The  sporangia  are  borne  by  the  leaves,  generally  upon 
the  under  surface,  and  are  usually  closely  associated  with 
the  veins  and  organized  into  groups  of  definite  form,  known 
as  sort.  A  sorus  may  be  round  or  elongated,  and  is  usually 
covered  by  a  delicate  flap  (indusium)  which  arises  from  the 
epidermis  (Figs.  118,  123,  124).  Occasionally  the  sori  are 
extended  along  the  under  surface  of  the  margin  of  the  leaf, 
as  in  maidenhair  fern  (Adiantum),  and  the  common  brake 
(Pteris),  in  which  case  they  are  protected  by  the  inrolled 


Fig.  12.3.    Fragrant  shield  fern  (.Aspid- 

ium  fragrans),  showing  general 
habit,  and  to  the  left  (a)  the  under 
surface  of  a  leaflet  bearing  sori 
covered  by  shield-like  indusia.— 
After  Marion  Satterlee. 


Fig.  124.  The  bladder  fern  ( Cyst&ptt  its  bulb- 
ifera),  showing  general  habit,  and  to  the 
right  (a)  the  under  surface  of  a  leaflet, 
showing  the  dichotomous  venation,  and 
five  sori  protected  by  pouch-like  indusia. 
—After  Marion  Satterlee. 


PTEKLDOPHYTES 


145 


margin  (Figs.  125,  126),  which  may  be  called  a  "false  in- 
dusium." 

It  is  evident  that  such  leaves  are  doing  two  distinct 
kinds  of  work — chlorophyll  work  and  spore  formation. 
This  is  true  of  most  of  the  ordinary  Ferns,  but  some  of 
them  show  a  tendency  to  di- 
vide the  work.  Certain  leaves, 
or  certain  leaf-branches,  pro- 
duce spores  and  do  no  chloro- 
phyll work,  while  others  do 
chlorophyll  work  and  produce 
no  spores.  This  differentia- 
tion in  the  leaves  or  leaf-re- 
gions is  indicated  by  appro- 
priate names.  Those  leaves 
which  produce  only  spores  are 
called  sporophylls,  meaning 
"spore  leaves,"  while  the  leaf 
branches  thus  set  apart  are 
called  sporophyll  branches. 
Those  leaves  which  only  do 
chlorophyll  work  are  called  fo- 
liage leaves  ;  and  such  branch- 
es are  foliage  branches.  As 
sporophylls  are  not  called  upon 
for  chlorophyll  work  they  often 

become  much  modified,  being  much  more  compact,  and  not 
at  all  resembling  the  foliage  leaves.  Such  a  differentiation 
may  be  seen  in  the  ostrich  fern  and  sensitive  fern  ( Onoclea) 
(Figs.  127,  128),  the  climbing  fern  (Lygodium),  the  royal 
fern  (Osmunda),  the  moonwort  (Botrycliium)  (Fig.  129), 
and  the  adder's  tongue  {Ophioglossum)  (Fig.  130). 

An  ordinary  fern  sporangium  consists  of  a  slender  stalk 
and  a  bulbous  top  which  is  the  spore  case  (Fig.  118,  6). 
This  case  has  a  delicate  wall  formed  of  a  single  layer  of 
cells,  and  extending  around  it  from  the  stalk  and  nearly  to 


Fig.  135.  Leaflets  of  two  common 
ferns :  A,  the  common  brake 
{Pterin);  B,  maidenhair  (Adian- 
tum);  both  showing  sori  borne 
at  the  margin  and  protected  by 
the  infolded  margin,  which  thus 
forms  a  false  indusium. — Cald- 
well. 


146  PLANT  STRUCTDKES 

the  stalk  again,  like  a  meridian  line  about  a  globe,  is  a  row  of 
peculiar  cells  with  thick  walls,  forming  a  heavy  ring,  called 
the  annitlus.  The  annulus  is  like  a  bent  spring,  and  when 
the  delicate  wall  becomes  yielding  the  spring  straightens 
violently,  the  wall  is  torn,  and  in  the  recoil  the  spores  are 
discharged  with  considerable  force  (Fig.  131).     This  dis- 


Fig.  126.— The  purple  cliff  brake  (Pelkea  atropurpurea),  showing  general  habit,  and 
at  a  a  single  leaflet  showing  the  dichotomous  venation  and  the  infolded  margin 
covering  the  eori.— After  Marion  Satterlee. 

charge  of  fern  spores  may  be  seen  by  placing  some  sporangia 
upon  a  moist  slide,  and  under  a  low  power  watching  them 
as  they  dry  and  burst. 

Within  this  sporangium  the  archesporium  (see  §  66) 
consists  of  a  single  cell,  which  by  division  finally  produces 


PTEEIDOPHYTES 


147 


numerous  mother  cells,  in  each  of  which  a  tetrad  of  spores 
is  formed.     The  disorganization  of  the  walls  of  the  mother 


sUsLAMAJV 

Fig.  127.    The  ostrich  fern  ( Onoctea  strutkiopteris),  showing  differentiation  of  foliage 
leaf  (a)  and  sporophyll  (6).—  After  Marion  Satterlee. 

cells  sets  the  spores  free  in  the  cavity  of  the  sporangium, 
and  ready  for  discharge. 


Fig.  128.    The  sensitive  fern  (Onoclea  senHbilis),  showing  differentiation  of  foliage 
leaves  and  sporophylls. — From  "Field,  Forest,  and  Wayside  Flowers." 


PTERIDOPHYTES 


149 


Among  the  Bryophytes  the  sporogenous  tissue  appears 
very  early  in  the  development  of  the  sporogonium,  the  pro- 
duction of  spores  being  its  only  function ;  also  there  is  a 


At 


^ 


^ 


Fig.  129.  A  moonwort  (Botrychi- 
tiDi),  showing  the  leaf  differen- 
tiated into  foliage  and  sporophyll 
branches.— After  Strasburger. 


Fig.  130.  The  adder's  tongue  (Ophiogloamm 
vulgatum),  showing  two  leaves,  each 
with  a  foliage  branch  and  a  much  longer 
sporophyll  branch.— After  Marion  Sat- 

TERLEE. 


150 


PLANT   STRUCT ORES 


tendency  to  restrict  the  sporogenous  tissue  and  increase  the 
sterile  tissue.  It  will  be  observed  that  with  the  introduc- 
tion of  the  leafy  sporophyte  among  the  Pteridophytes  the 
sporangia  appear  much  later  in  its  development,  sometimes 
not  appearing  for  several  years,   as  though   they  are  of 


Fig.  131.  A  series  showing  the  dehiscence  of  a  fern  sporangium,  the  rupture  of  the 
wall,  the  straightening  anil  bending  back  of  the  annulus,  and  the  recoil.— After 
Atkinson. 


secondary  importance  as  compared  with  chlorophyll  work  ; 
and  that  the  sporogenous  tissue  is  far  more  restricted,  the 
sporangia  forming  a  very  small  part  of  the  bulk  of  the 
sporophyte  body. 


PTERIDOPHYTES  15} 

81.  Heterospory. — This  phenomenon  appears  first  among 
Pteridophytes,  but  it  is  not  characteristic  of  them,  being  en- 
tirely absent  from  the  true  Ferns,  which  far  outnumber  all 
other  Pteridophytes.  Its  chief  interest  lies  in  the  fact  that 
it  is  universal  among  the  Spermatophytes,  and  that  it  rep- 
resents the  change  which  leads  to  the  appearance  of  that 
high  group.  It  is  impossible  to  understand  the  greatest 
group  of  plants,  therefore,  without  knowing  something 
about  heterospory.  As  it  begins  in  simple  fashion  among 
Pteridophytes,  and  is  probably  the  greatest  contribution 
they  have  made  to  the  evolution  of  the  plant  kingdom, 
unless  it  be  the  leafy  sporophyte,  it  is  best  explained  here. 

In  the  ordinary  Ferns  all  the  spores  in  the  sporangia 
are  alike,  and  when  they  germinate  each  spore  produces  a 
prothallium  upon  which  both  antheridia  and  archegonia 
appear.  It  has  been  remarked,  however,  that  some  pro- 
thallia  are  dioecious — that  is,  some  bear  only  antheridia 
and  others  bear  only  archegonia.  In  this  case  it  is  evident 
that  the  spores  in  the  sporangium,  although  they  may  ap- 
pear alike,  produce  different  kinds  of  prothallia,  which 
may  be  called  male  and  female,  as  each  is  distinguished  by 
the  sex  organ  which  it  produces.  As  archegonia  are  only 
produced  by  well-nourished  prothallia,  it  seems  fair  to  sup- 
pose that  the  larger  spores  will  produce  female  prothallia, 
and  the  smaller  ones  male  prothallia. 

This  condition  of  things  seems  to  have  developed  finally 
into  a  permanent  and  decided  difference  in  the  size  of  the 
spores,  some  being  quite  small  and  others  relatively  large, 
the  small  ones  producing  male  gametophytes  (prothallia 
with  antheridia),  and  the  large  ones  female  gametophytes 
(prothallia  with  archegonia).  When  asexual  spores  differ 
thus  permanently  in  size,  and  give  rise  to  gametophytes  of 
different  sexes,  we  have  the  condition  called  heterospory 
("  spores  different "),  and  such  plants  are  called  heterospo- 
rous  (Fig.  139).  In  contrast  with  heterosporous  plants, those 
in  which  the  asexual  spores  appear  alike  are  called  homos- 


152  PLANT  STRUCTURES 

porous,  or  sometimes  isosporous,  both  terms  meaning 
"spores  similar."  The  corresponding  noun  form  is  liomos- 
pory  or  isospory.  Bryophytes  and  most  Pteridophytes  are 
homosporous,  while  some  Pteridophytes  and  all  Spermato- 
phytes  are  heterosporous. 

It  is  convenient  to  distinguish  by  suitable  names  the 
two  kinds  of  asexual  spores  produced  by  the  sporangia  of 
heterosporous  plants  (Fig.  139).  The  large  ones  are  called 
megaspores,  or  by  some  writers  macrospores,  both  terms 
meaning  " large  spores";  the  small  ones  are  called  micro- 
spores, or  "  small  spores."  It  should  be  remembered  that 
megaspores  always  produce  female  gametophytes,  and  mi- 
crospores male  gametophytes. 

This  differentiation  does  not  end  with  the  spores,  but 
soon  involves  the  sporangia  (Fig.  139).  Some  sporangia 
produce  only  megaspores,  and  are  called  megasporangia ; 
others  produce  only  microspores,  and  are  called  microspo- 
rangia. It  is  important  to  note  that  while  microsporangia 
usually  produce  numerous  microspores,  the  megasporangia 
produce  much  fewer  megaspores,  the  tendency  being  to 
diminish  the  number  and  increase  the  size,  until  finally 
there  are  megasporangia  which  produce  but  a  single  large 
functioning  megaspore. 

The  differentiation  goes  still  further.  If  the  sporangia 
are  born  upon  sporophylls,  the  sporophylls  themselves  may 
differentiate,  some  bearing  only  megasporangia,  and  others 
only  microsporangia,  the  former  being  called  megasporo- 
phylls,  the  latter  microsporophylls.  In  such  a  case  the 
sequence  is  as  follows  :  megasporophylls  produce  megaspo- 
rangia, which  produce  megaspores,  which  in  germination 
produce  the  female  gametophytes  (prothallia  with  archego- 
nia) ;  while  the  microsporophylls  produce  microsporangia, 
which  produce  microspores,  which  in  germination  produce 
male  gametophytes  (prothallia  with  antheridia). 

A  formula  may  indicate  the  life  history  of  a  heteros- 
porous plant.     The  formula  of  homosporous  plants  with 


PTER1D0PHYTES 


153 


alternation  of  generations  (Bryophytes  and  most  Pterido- 
phytes)  was  given  as  follows  (§  62)  : 

G=g  >  o— S— o— G=8  >  o— S— o— G=g  >  o— S,  etc. 

In  the  case  of  heterosporous  plants  (some  Pteridophytes 
and  all  Spermatophytes)  it  would  be  modified  as  follows  : 
S=8>o— S=8=g=g>o— S=8=g=8>o— S,  etc. 

In  this  case  two  gametophytes  are  involved,  one  pro- 
ducing a  sperm,  the  other  an  egg,  which  fuse  and  form  the 
oospore,  which  in  germination  produces  the  sporophyte, 
which  produces  two  kinds  of  asexual  spores  (megaspores 
and  microspores),  which  in  germination  produce  the  two 
gametophytes  again. 

One  additional  fact  connected  with  heterospory  should 
be  mentioned,  and  that  is  the  great  reduction  of  the  gam- 
etophyte.  In  the  homosporous  ferns  the  spore  develops 
a  small  but  free  and  independent  prothallium  which  pro- 
duces both  sex  organs.  When  in  heterosporous  plants  this 
work  of  producing  sex  organs  is  divided  between  two  gam- 
etophytes they  become  very  much  reduced  in  size  and  lose 
their  freedom  and  independence.  They  are  so  small  that 
they  do  not  escape  entirely,  if  at  all,  from  the  embrace  of 
the  spores  which  produce  them,  and  are  mainly  dependent 
for  their  nourishment  upon  the  food  stored  up  in  the  spores 
(Figs.  140,  141).  As  the  spore  is  produced  by  the  sporo- 
phyte, heterospory  brings  about  a  condition  in  which  the 
gametophyte  is  dependent  upon  the  sporophyte,  an  exact 
reversal  of  the  condition  in  Bryophytes. 

The  relative  importance  of  the  gametophyte  and  the 
sporophyte  throughout  the  plant  kingdom  may  be  roughly 
indicated  by  the  accompanying   diagram,  in   which  the 


shaded  part  of  the  parallelogram  represents  the  gameto- 
phyte and  the  unshaded  part  the  sporophyte.     Among  the 


154 


PLANT   STKUCTUKES 


lowest  plants  the  gametophyte  is  represented  by  the  whole 
plant  structure.  When  the  sporophyte  first  appears  it  is 
dependent  upon  the  gametophyte  (some  Thallophytes  and 
the  Bryophytes),  and  is  relatively  inconspicuous.  Later 
the  sporophyte  becomes  independent  (most  Pteridophytes), 
the  gametophyte  being  relatively  inconspicuous.  Finally 
(heterosporous  Pteridophytes)  the  gametophyte  becomes 
dependent  upon  the  sporophyte,  and  in  Spermatophytes  is 
so  inconspicuous  and  concealed  that  it  is  only  observed  by 
means  of  laboratory  appliances,  while  the  sporophyte  is  the 
whole  plant  of  ordinary  observation. 


CHAPTER  X 

THE  GREAT  GROUPS  OF  PTERIDOPHYTES 

82.  The  great  groups. — At  least  three  independent  lines 
of  Pteridophytes  are  recognized  :  (1)  Filicales  (Ferns), 
(2)  Equisetales  (Scouring  rushes,  Horsetails),  and  (3)  Ly- 
copodiales  (Club-mosses).  The  Ferns  are  much  the  most 
abundant,  the  Club-mosses  are  represented  by  a  few  hun- 
dred forms,  while  the  Horsetails  include  only  about  twenty- 
five  species.  These  three  great  groups  are  so  unlike  that 
they  hardly  seem  to  belong  together  in  the  same  division 
of  the  plant  kingdom. 

Filicales  {Ferns) 

83.  General  characters. — The  Ferns  were  used  in  the 
preceding  chapter  as  types  of  Pteridophytes,  so  that  little 
need  be  added.  They  well  deserve  to  stand  as  types,  as 
they  contain  about  four  thousand  of  the  four  thousand  five 
hundred  species  belonging  to  Pteridophytes.  Although 
found  in  considerable  numbers  in  temperate  regions,  their 
chief  display  is  in  the  tropics,  where  they  form  a  striking 
and  characteristic  feature  of  the  vegetation.  In  the  trop- 
ics not  only  are  great  masses  of  the  low  forms  to  be  seen, 
from  those  with  delicate  and  filmy  moss  like  leaves  to  those 
with  huge  leaves,  but  also  tree  forms  with  cylindrical 
trunks  encased  by  the  rough  remnants  of  fallen  leaves  and 
sometimes  rising  to  a  height  of  thirty-five  to  forty-five 
feet,  with  a  great  crown  of  leaves  fifteen  to  twenty  feet 
long  (Fig.  120). 

155 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES      157 

There  are  also  epiphytic  forms  (air  plants) — that  is, 
those  which  perch  "  upon  other  plants  "  but  derive  no 
nourishment  from  them  (Fig.  112).  This  habit  belongs 
chiefly  to  the  warm  and  moist  tropics,  where  the  plants 
can  absorb  sufficient  moisture  from  the  air  without  send- 
ing roots  into  the  soil.  In  this  way  many  of  the  tropical 
ferns  are  found  growing  upon  living  and  dead  trees  and 
other  plants.  In  the  temperate  regions  the  chief  epi- 
phytes are  Lichens,  Liverworts,  and  Mosses,  the  Ferns  be- 
ing chiefly  found  in  moist  woods  and  ravines  (Fig.  132), 
although  a  number  grow  in  comparatively  dry  and  exposed 
situations,  sometimes  covering  extensive  areas,  as  the  com- 
mon brake  (Pteris)  (Fig.  125). 

The  Filicales  differ  from  the  other  groups  of  Pterido- 
phytes  chiefly  in  having  few  large  leaves,  which  do  chloro- 
phyll work  and  bear  sporangia.  In  a  few  of  them  there  is  a 
differentiation  of  functions  in  foliage  branches  and  sporo- 
phyll  branches  (Figs.  127-130),  but  even  this  is  excep- 
tional. Another  distinction  is  that  the  stems  are  un- 
branched. 

84.  Origin  of  sporangia. — An  important  feature  in  the 
Ferns  is  the  origin  of  the  sporangia.  In  some  of  them  a 
sporangium  is  developed  from  a  single  epidermal  cell  of 
the  leaf,  and  is  an  entirely  superficial  and  generally  stalked 
affair  (Fig.  118,  5)  ;  in  others  the  sporangium  in  its  devel- 
opment involves  several  epidermal  and  deeper  cells  of  the 
leaf,  and  is  more  or  less  of  an  imbedded  affair.  In  the  first 
case  the  ferns  are  said  to  be  leptosporangiate  ;  in  the  sec- 
ond case  they  are  eusporangiate. 

The  leptosporangiate  Ferns  are  overwhelmingly  abun- 
dant as  compared  with  the  Eusporangiates.  Back  in  the 
Coal-measures,  however,  there  was  an  abundant  fern  vege- 
tation which  was  probably  all  eusporangiate.  The  Lep- 
tosporangiates  seem  to  be  the  modern  Ferns,  the  once 
abundant  Eusporangiates  being  represented  now  in  the 
temperate  regions  only  by  such  forms  as  moonwort  (Bo- 
29 


158 


PLANT  STKDCTUKES 


trychium)  (Fig.  129)  and  adder's  tongue  (Ophioglossum) 
(Fig.  130).  It  is  important  to  note,  however,  that  the 
Horsetails  and  Club-mosses  are  Eusporangiates,  as  well  as 
all  the  Seed-plants. 

Another  small  but  interesting  group  of  Ferns  includes 
the  "Water-ferns,"  floating  forms  or  sometimes  on  muddy 
flats.     The  common  Marsilia  may  be  taken  as  a  type  (Fig. 

133).  The  slender  creeping  stem 
sends  down  numerous  roots  into 
the  mucky  soil,  and  at  intervals 
gives  rise  to  a  comparatively  large 
leaf.  This  leaf  has  a  long  erect 
petiole  and  a  blade  of  four  spread- 


••      ■,    ft     HIV 


Fig.  133. — A  water-fern  (Marsilia), 
showing  horizontal  stem,  with 
descending  roots,  and  ascend- 
ing leaves ;  a,  a  yonng  leaf 
showing  circinate  vernation  ; 
s,«,sporophyll  branches  ("spo- 
rocarps  ").—  After  Bischofp. 


J? 

Fio.  134.  One  of  the  floating  water-ferns  (Sal- 
vinia),  showing  side  view  (.4)  and  view  from 
above  (2?).  The  dangling  root-like  processes 
are  the  modified  submerged  leaves.  In  A, 
near  the  top  of  the  cluster  of  submerged 
leaves,  some  sporophyll  branches  ("sporo- 
carps")  may  be  seen. — After  Bischofp. 


ing  wedge-shaped  leaflets  like  a  "four-leaved  clover." 
dichotomous  venation  and  circinate  vernation  at  once 
gest  the  fern  alliance.     From  near  the  base  of  the  petiole 


The 
sug- 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES      159 

another  leaf  branch  arises,  in  which  the  blade  is  modified 
as  a  sporophyll.  In  this  case  the  sporophyll  incloses  the 
sporangia  and  becomes  hard  and  nut-like.  Another  com- 
mon form  is  the  floating  Salvinia  (Fig.  134).  The  chief 
interest  lies  in  the  fact  that  the  water-ferns  are  heteros- 
porous.  As  they  are  leptosporangiate  they  are  thought 
to  have  been  derived  from  the  ordinary  leptosporangiate 
Ferns,  which  are  homosporous. 

Three  fern  groups  are  thus  outlined  :  (1)  homosporous- 
eusporangiate  forms,  now  almost  extinct ;  (2)  homosporous- 
leptosporangiate  forms,  the  great  overwhelming  modern 
group,  not  only  of  Filicales  but  also  of  Pteridophytes,  well 
called  true  Ferns,  and  thought  to  be  derived  from  the  pre- 
ceding group ;  and  (3)  heterosporous-leptosporangiate 
forms,  the  water-ferns,  thought  to  be  derived  from  the  pre- 
ceding group. 

Equisetales  ( Horsetails  or  Scouring  rushes) 

85.  General  characters. — The  twenty-five  forms  now  rep- 
resenting this  great  group  belong  to  a  single  genus  (Equise- 
ttim,  meaning  "  horsetail  "),  but  they  are  but  the  linger- 
ing remnants  of  an  abundant  flora  which  lived  in  the  time 
of  the  Coal-measures,  and  helped  to  form  the  forest  vegeta- 
tion. The  living  forms  are  small  and  inconspicuous,  but 
very  characteristic  in  appearance.  They  grow  in  moist  or 
dry  places,  sometimes  in  great  abundance  (Fig.  135). 

The  stem  is  slender  and  conspicuously  jointed,  the  joints 
separating  easily ;  it  is  also  green  and  fluted  with  small 
longitudinal  ridges  ;  and  there  is  such  an  abundant  deposit 
of  silica  in  the  epidermis  that  the  plants  feel  rough.  This 
last  property  suggested  its  former  use  in  scouring,  and  its 
name  "  scouring  rush."  At  each  joint  is  a  sheath  of  minute 
leaves,  more  or  less  coalesced,  the  individual  leaves  some- 
times being  indicated  only  by  minute  teeth.  This  arrange- 
ment of  leaves  in  a  circle  about  the  joint  is  called  the  cyclic 


Fig.  135.  Equisetum  arvense,  a  common  horsetail:  /,  three  fertile  shoots  rising  from 
the  dorsiventral  stem,  showing  the  cycles  of  coalesced  scale-leaves  at  the  joints 
and  the  terminal  strobili  with  numerous  sporophylls,  that  at  a  being  mature;  2, 
a  sterile  shoot  from  the  same  stem,  showing  branching;  3,  a  single  peltate  sporo- 
phyll  bearing  sporangia;  U,  view  of  sporophyll  from  beneath,  showing  dehiscence 
of  sporangia;  5,  6,  7.  spores,  showing  the  unwinding  of  the  outer  coat,  which  aids 
in  dispersal. —After  Wossidlo. 


THE  GEEAT  GROUPS  OE  PTERIDOPHYTES 


161 


arrangement,  or  sometimes  the  whorled  arrangement,  each 
such  set  of  leaves  being  called  a  cycle  or  a  whorl.  These 
leaves  contain  no  chlorophyll  and  have  evidently  abandoned 
chlorophyll  work,  which  is  carried  on  by  the  green  stem. 
Such  leaves  are  known  as  scales,  to  distinguish  them  from 
foliage  leaves.  The  stem  is  either  simple  or  profusely 
branched  (Fig.  135). 

86.  The  strobilus. — One  of  the  distinguishing  characters 
of  the  group  is  that  chlorophyll-work  and  spore-formation 
are  completely  differentiated.     Although  the  foliage  leaves 


Fig.  136.  Dicecious  gainetophytes  of  Equisetum :  A,  the  female  gametophyte,  show- 
ing branching,  rhizolds,  and  an  archegonium  {ar)\  B,  the  male  gametophyte, 
showing  several  antheridia  ( <5 ).— After  Campbell. 


are  reduced  to  scales,  and  the  chlorophyll-work  is  done  by 
the  stem,  there  are  well-organized  sporophylls.  The  sporo- 
phylls  are  grouped  close  together  at  the  end  of  the  stem  in 
a  compact  conical  cluster  which  is  called  a  strobilus,  the 
Latin  name  for  "pine  cone,"  which  this  cluster  of  sporor 
phylls  resembles  (Fig.  135). 

Each  sporophyll  consists  of  a  stalk-like  portion  and  a 
shield-like  (peltate)  top.      Beneath  the  shield  hang  the 


162  PLANT  STKDCTUKES 

sporangia,  which  produce  spores  of  but  one  kind,  hence 
these  plants  are  homosporous  ;  and  as  the  sporangia  origi- 
nate in  eusporangiate  fashion,  Equisetum  has  the  homospo- 
rous-eusporangiate  combination  shown  by  one  of  the  Fern 
groups.  It  is  interesting  to  know,  however,  that  some  of 
the  ancient,  more  highly  organized  members  of  this  group 
were  heterosporous,  and  that  the  present  forms  have 
dioecious  gametophytes  (Fig.  136). 

Lycopodiales  ( Club-mosses) 

87.  General  characters. — This  group  is  now  represented 
by  about  five  hundred  species,  most  of  which  belong  to 
the  two  genera  Lycopodium  and  Selaginella,  the  latter 
being  much  the  larger  genus.  The  plants  have  slender, 
branching,  prostrate,  or  erect  stems  completely  clothed 
with  small  foliage  leaves,  having  a  general  moss-like 
appearance  (Fig.  137).  Often  the  erect  branches  are 
terminated  by  conspicuous  conical  or  cylindrical  strobili, 
which  are  the  "  clubs  "  that  enter  into  the  name  ".  Club- 
mosses."  There  is  also  a  certain  kind  of  resemblance 
to  miniature  pines,  so  that  the  name  "  Ground-pines "  is 
sometimes  used. 

Lycopodiales  were  once  much  more  abundant  than  now, 
and  more  highly  organized,  forming  a  conspicuous  part  of 
the  forest  vegetation  of  the  Coal-measures. 

One  of  the  distinguishing  marks  of  the  group  is  that  the 
sperm  does  not  resemble  that  of  the  other  Pteridophytes, 
but  is  of  the  Bryophyte  type  (Fig.  140,  F).  That  is,  it 
consists  of  a  small  body  with  two  cilia,  instead  of  a  large 
spirally  coiled  body  with  many  cilia.  Another  distinguish- 
ing character  is  that  there  is  but  a  single  sporangium  pro- 
duced by  each  sporophyll  (Fig.  137).  This  is  in  marked 
contrast  with  the  Filicales,  whose  leaves  bear  very  numer- 
ous sporangia,  and  with  the  Equisetales,  whose  sporophylls 
bear  several  sporangia. 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES 


163 


Fig.  137.  A  common  club-moss  {Lycopodimn  clavatum):  1,  the  whole  plant,  showing 
horizontal  stem  giving  rise  to  roots  and  to  erect  branches  bearing  strobili;  2,  a 
single  sporophyll  with  its  sporangium;  3,  spores,  much  magnified.— After  Wos- 


88.  Lycopodimn. — This  genus  contains  fewer  forms  than 
the  other,  but  they  are  larger  and  coarser  and  more  charac- 
teristic of  the  temperate  regions,  being  the  ordinary  Club- 
mosses  (Fig.  137).  They  also  more  commonly  display 
conspicuous  and  distinct  strobili,  although  there  is  every 


164 


PLANT  STRUCTUKES 


gradation  between  ordinary  foliage  leaves  and  distinct 
sporophylls. 

The  sporangia  are  borne  either  by  distinct  sporophylls 
or  by  the  ordinary  foliage  leaves  near  the  summit  of  the 
stem.  At  the  base  of  each  of  these  leaves,  or  sporophylls, 
on  the  upper  side,  is  a  single  sporangium  (Fig.  137).  The 
sporangia  are  eusporangiate  in  origin,  and  as  the  spores  are 
all  alike,  Lycopodium  has  the  same  homosporous-eusporan- 
giate  combination  noted  in  Equisetales  and  in  one  of  the 
groups  of  Filicales. 

89.  Selaginella. — This  large  genus  contains  the  smaller, 
more  delicate  Club-mosses,  often  being  called  the  "  little 
Club-mosses."    They  are  especially  displayed  in  the  trop- 


Fia.  138.    Selaginella,  showing  general  spray-like  habit,  and  dangling  leafless  stems 
which  strike  root  (rhizophores). — From  "  Plant  Relations.1' 


ics,  and  are  common  in  greenhouses  as  delicate,  mossy, 
decorative  plants  (Fig.  138).  In  general  the  sporophylls 
are  not  different  from  the  ordinary  leaves  (Fig.  139),  but 
sometimes  they  are  modified,  though  not  so  distinct  as  in 
certain  species  of  Lycopodium 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES 


165 


The  solitary  sporangium  appears  in  the  axils  (upper 
angles  formed  by  the  leaves  with  the  stem)  of  the  leaves 
and  sporophylls,  but  arise  from  the  stem  instead  of  the 


Fig.  139.  Sdaginella  Martensii:  A,  branch  bearing  strobili;  B,  a  microsporophyll 
with  a  microsporangium,  showing  microspores  through  a  rupture  in  the  wall;  C, 
a  megasporophyll  with  a  megasporangium ;  D,  megaspores  :  E,  microspores.— 

GOLDBEKGER. 


166 


PLANT  STRUCTURES 


leaf  (Fig.  139).  This  is  important  as  showing  that  sporan- 
gia may  be  produced  by  stems  as  well  as  by  leaves,  those 
being  produced  by  leaves  being  called  foliar,  and  those  by 
stem  cauline. 

The  most  important  fact  in  connection  with  Selaginella, 
however,  is  that  it  is  heterosporous.  Megasporangia,  each 
usually  containing  but  four  megaspores,  are  found  in  the 
axils  of  a  few  of  the  lower  leaves  of  the  strobilus,  and  more 
numerous  microsporangia  occur  in  the  upper  axils,  con- 
taining very  many  microspores  (Fig.  139).  The  character 
of  the  gametophytes  of  heterosporous  Pteridophytes  may 
be  well  illustrated  by  those  of  Selaginella. 

The  microspore  germinates  and  forms  a  male  gameto- 
phyte  so  small  that  it  is  entirely  included  within  the  spore 


w  J0 


Fig.  140.  Male  gametophyte  of  Selaginella:  in  each  case  p  is  the  prothallial  cell,  w 
the  wall  cells  of  the  antheridium,  s  the  sperm  tissue:  F,  the  biciliate  sperms.— 
After  Bklajepp. 


wall  (Fig.  140).  A  single  small  cell  is  all  that  represents 
the  ordinary  cells  of  the  prothallium,  while  all  the  rest  is 
an  antheridium,  consisting  of  a  wall  of  a  few  cells  sur- 
rounding numerous  sperm  mother  cells.     In  the  presence 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES 


167 


of  water  the  antheridium  wall  breaks  down,  as  also  do  the 
walls  of  the  mother  cells,  and  the  small  biciliate  sperms 
are  set  free. 

The  much  larger  megaspores  germinate  and  become 
filled  with  a  mass  of  numerous  nutritive  cells,  representing 
the  ordinary  cells  of  a  prothallium  (Fig.  141).  The  spore 
wall  is  broken  by  this  growing  prothallium,  a  part  of  which 
thus  protrudes  and  becomes  exposed,  although  the  main 
part  of  it  is  still  invested  by  the  old  megaspore  wall.  In 
this  exposed  portion 
of  the  female  gameto- 
phyte  the  archegonia 
appear,  and  thus  be- 
come accessible  to  the 
sperms.  In  the  case 
of  Isoetes  (see  §  90) 
the  reduction  of  the 
female  gametophyte  is 
even  greater,  as  it  does 
not  project  from  the 
megaspore  wall  at  all, 
and  the  archegonia 
are  made  accessible 
through  cracks  in  the 
wall  immediately  over 
them. 

The  embryo  of  Se- 
laginella  is  also  impor- 
tant to  consider.  Be- 
ginning its  development  in  the  venter  of  the  archegonium, 
it  first  lies  upon  the  exposed  margin  of  the  prothallium, 
while  the  mass  of  nutritive  cells  lie  deep  within  the  mega- 
spore (Fig.  141,  embv  embj.  It  first  develops  an  elongated 
cell,  or  row  of  cells,  which  thrusts  the  embryo  cell  deeper 
among  the  nutritive  cells.  This  cell  or  row  of  cells,  formed 
by  the  embryo  to  place  the  real  embryo  cell  in  better  rela- 


spm 


Pig.  141.  Female  gametophyte  of  a  Selaginella  : 
spm,  wall  of  megaspore ;  pr,  gametophyte ; 
ar,  an  archegonium ;  embx  and  emb^  em- 
bryo sporophytes  ;  et,  suspensors  ;  the  gam- 
etophyte has  developed  a  few  rhizoids.— 
After  Pfepfeb. 


168 


PLANT   STRUCTURES 


tion  to  its  food  supply,  is  called  the  suspensor,  and  is  a 
temporary  organ  of  the  embryo  (Figs.  141,  142,  et).  At 
the  end  of  the  suspensor  the  real  embryo  develops,  and 
when  its  regions  become  organized  it  shows  the  following 
parts  :  (1)  a  large  foot  buried  among  the  nutritive  cells  of 
the  prothallium  and  absorbing  nourishment ;  (2)  a  root 
stretching  out  toward  the  substratum  ;  (3)  a  stem  extend- 


Fig.  142.  Embryo  of  Selaginella  removed  from  the  gametophyte,  showing  suspensor 
(et),  root-tip  (w),  foot  (/),  cotyledons  (bl),  stem-tip  {sty,  and  ligules  (lig).—  After 
Pfeffer. 


ing  in  the  other  direction,  and  bearing  just  behind  its  tip 
(4)  a  pair  of  opposite  leaves  (cotyledons)  (Fig.  142). 

As  the  sporangia  of  Selaginella  are  eusporangiate,  this 
genus  has  the  heterosporous-eusporangiate  combination — a 
combination  not  mentioned  heretofore,  and  being  of  special 
interest  as  it  is  the  combination  which  belongs  to  all  the 
Spermatophytes.  For  this  and  other  reasons,  Selaginella 
is  one  of  the  Pteridophyte  forms  which  has  attracted 
special  attention,  as  possibly  representing  one  of  the  an- 
cestral forms  of  the  Seed-plants. 


THE  GREAT  GROUPS  OF  PTERIDOPHYTES 


169 


90.  Isoetes. — This  little  group  of  aquatic  plants,  known 
as  "quillworts,"  is  very  puzzling  as  to  its  relationships 
among  Pteridophytes.  By  some  it  is  put  with  the  Ferns, 
forming  a  distinct  division  of  Filicales  ;  by  others  it  is  put 


Fig.  143.  A  common  quillwort  (Isoetes  lacus- 
tris),  showing  cluster  of  roots  dichoto- 
mously  branching,  and  cluster  of  leaves 
each  enlarged  at  base  and  inclosing  a  sin- 
gle sporangium.— After  Schenck. 


Fig.  144.  Sperm  of  Isoetes,  show- 
ing spiral  body  and  seven  long 
cilia  arising  from  the  beak. — 
After  Belajeff. 


with  the  Club-mosses,  and  is  associated  with  Selaginella. 
It  resembles  a  bunch  of  fine  grass  growing  in  shoal  water 
or  in  mud,  but  the  leaves  enlarge  at  the  base  and  overlap 
one  another  and  the  very  short  tuberous  stem  (Fig.  143). 
Within  each  enlarged  leaf  base  a  single  sporangium  is 
formed,  and  the  cluster  contains  both  megasporangia  and 
microsporangia.  The  sporangia  are  eusporangiate,  and 
therefore  Isoetes  shares  with  Selaginella  the  distinction  of 


^70  PLANT  STRUCTURES 

having  the  heterosporous-eusporangiate  combination,  which 
is  a  feature  of  the  Seed-plants. 

The  embryo  is  also  peculiar,  and  is  so  suggestive  of  the 
embryo  of  the  Monocotyledons  (see  §  114)  among  Seed- 
plants  that  some  regard  it  as  possibly  representing  the 
ancestral  forms  of  that  group  of  Spermatophytes.  The 
peculiarity  lies  in  the  fact  that  at  one  end  of  the  axis  of  the 
embryo  is  a  root,  and  at  the  other  the  first  leaf  (cotyledon), 
while  the  stem  tip  rises  as  a  lateral  outgrowth.  This  is 
exactly  the  distinctive  feature  of  the  embryo  of  Monocoty- 
ledons. 

The  greatest  obstacle  in  the  way  of  associating  these 
quillworts  with  the  Club-mosses  is  the  fact  that  their  sperms 
are  of  the  large  and  spirally  coiled  multiciliate  type  which 
belongs  to  Filicales  and  Equisetales  (Fig.  144),  and  not  at 
all  the  small  biciliate  type  which  characterizes  the  Club- 
mosses  (Fig.  140).  To  sum  up,  the  short  unbranched  stem 
with  comparatively  few  large  leaves,  and  the  coiled  multi- 
ciliate sperm,  suggest  Filicales ;  while  the  solitary  spo- 
rangia and  the  heterosporous-eusporangiate  character  sug- 
gest Selaginella. 


CHAPTER   XI 

SPERMATOPHYTES :    GYMNOSPERMS 

91.  Summary  from  Pteridophytes. — In  considering  the 
important  contributions  of  Pteridophytes  to  the  evolution 
of  the  plant  kingdom  the  following  seem  worthy  of  note  : 

(1)  Prominence  of  sporophyte  and  development  of  vascu- 
lar system. — This  prominence  is  associated  with  the  display 
of  leaves  for  chlorophyll  work,  and  the  leaves  necessitate 
the  work  of  conduction,  which  is  arranged  for  by  the  vas- 
cular system.     This  fact  is  true  of  the  whole  group. 

(2)  Differentiation  of  sporophylls. — The  appearance  of 
sporophylls  as  distinct  from  foliage  leaves,  and  their  or- 
ganization into  the  cluster  known  as  the  strobilus,  are  facts 
of  prime  importance.  This  differentiation  appears  more  or 
less  in  all  the  great  groups,  but  the  strobilus  is  distinct  only 
in  Horsetails  and  Club-mosses. 

(3)  Introduction  of  heterospory  and  reduction  of  gameto- 
pliytes. — Heterospory  appears  independently  in  all  of  the 
three  great  groups — in  the  water-ferns  among  the  Fili- 
cales,  in  the  ancient  horsetails  among  the  Equisetales,  and 
in  Selagifiella  and  Isoetes  among  Lycopodiales.  All  the 
other  Pteridophytes,  and  therefore  the  great  majority  of 
them,  are  homosporous.  The  importance  of  the  appear- 
ance of  heterospory  lies  in  the  fact  that  it  leads  to  the 
development  of  Spermatophytes,  and  associated  with  it  is 
a  great  reduction  of  the  gametophytes,  which  project  little, 
if  at  all,  from  the  spores  which  produce  them. 

92.  Summary  of  the  four  groups. — It  may  be  well  in  this 
connection  to  give  certain  prominent  characters  which  will 

171 


1^2  PLANT  STRUCTURES 

serve  to  distinguish  the  four  great  groups  of  plants.  It 
must  not  be  supposed  that  these  are  the  only  characters, 
or  even  the  most  important  ones  in  every  case,  but  they 
are  convenient  for  our  purpose.  Two  characters  are  given 
for  each  of  the  first  three  groups — one  a  positive  character 
which  belongs  to  it,  the  other  a  negative  character  which 
distinguishes  it  from  the  group  above,  and  becomes  the 
positive  character  of  that  group. 

(1)  Tliallopliytes. — Thallus  body,  but  no  archegonia. 

(2)  Bryophytes. — Archegonia,  but  no  vascular  system. 

(3)  Pteridophytes. — Vascular  system,  but  no  seeds. 

(4)  Spermatophytes. — Seeds. 

93.  General  characters  of  Spermatophytes. — This  is  the 
greatest  group  of  plants  in  rank  and  in  display.  So  con- 
spicuous are  they,  and  so  much  do  they  enter  into  our 
experience,  that  they  have  often  been  studied  as  "botany," 
to  the  exclusion  of  the  other  groups.  The  lower  groups 
are  not  meiely  necessary  to  fill  out  any  general  view  of  the 
plant  kingdom,  but  they  are  absolutely  essential  to  an 
understanding  of  the  structures  of  the  highest  group. 

This  great  dominant  group  has  received  a  variety  of 
names.  Sometimes  they  are  called  Anthophytes,  meaning 
"Flowering  plants,"  with  the  idea  that  they  are  distin- 
guished by  the  production  of  "  flowers."  A  flower  is  diffi- 
cult to  define,  but  in  the  popular  sense  all  Spermatophytes 
do  not  produce  flowers,  while  in  another  sense  the  strobilus 
of  Pteridophytes  is  a  flower.  Hence  the  flower  does  not 
accurately  limit  the  group,  and  the  name  Anthophytes  is 
not  in  general  use.  Much  more  commonly  the  group  is 
called  Phanerogams  (sometimes  corrupted  into  Phaenogams 
or  even  Phenogams),  meaning  "evident  sexual  reproduc- 
tion." At  the  time  this  name  was  proposed  all  the  other 
groups  were  called  Cryptogams,  meaning  "hidden  sexual 
reproduction."  It  is  a  curious  fact  that  the  names  ought 
to  have  been  reversed,  for  sexual  reproduction  is  much  more 
evident  in  Cryptogams  than  in  Phanerogams,  the  mistake 


SPERMATOCYTES:  GYMNOSPERMS  173 

arising  from  the  fact  that  what  were  supposed  to  be  sexual 
organs  in  Phanerogams  have  proved  not  to  be  such.  The 
name  Phanerogam,  therefore,  is  being  generally  abandoned  ; 
but  the  name  Cryptogam  is  a  useful  one  when  the  lower 
groups  are  to  be  referred  to ;  and  the  Pteridophytes  are 
still  very  frequently  called  the  Vascular  Crypt  ogams.  The 
most  distinguishing  mark  of  the  group  seems  to  be  the 
production  of  seeds,  and  hence  the  name  Spermatophytes, 
or  "  Seed-plants,"  is  coming  into  general  use. 

The  seed  can  be  better  defined  after  its  development 
has  been  described,  but  it  results  from  the  fact  that  in  this 
group  the  single  megaspore  is  never  discharged  from  its 
megasporangium,  but  germinates  just  where  it  is  devel- 
oped. The  great  fact  connected  with  the  group,  therefore, 
is  the  retention  of  the  megaspore,  which  results  in  a  seed. 
The  full  meaning  of  this  will  appear  later. 

There  are  two  very  independent  lines  of  Seed-plants, 
the  Gymnosperms  and  the  Anglo  sperms.  The  first  name 
means  "naked  seeds,"  referring  to  the  fact  that  the  seeds 
are  always  exposed ;  the  second  means  "  inclosed  seeds," 
as  the  seeds  are  inclosed  in  a  seed  vessel. 

Gymnosperms 

94.  General  characters. — The  most  familiar  Gymnosperms 
in  temperate  regions  are  the  pines,  spruces,  hemlocks, 
cedars,  etc.,  the  group  so  commonly  called  "evergreens." 
It  is  an  ancient  tree  group,  for  its  representatives  were 
associated  with  the  giant  club-mosses  and  horsetails  in 
the  forest  vegetation  of  the  Coal-measures.  Only  about 
four  hundred  species  exist  to-day  as  a  remnant  of  its  for- 
mer display,  although  the  pines  still  form  extensive  forests. 
The  group  is  so  diversified  in  its  structure  that  all  forms 
can  not  be  included  in  a  single  description.  The  common 
pine  (Pinus),  therefore,  will  be  taken  as  a  type,  to  show 
the  general  Gymnosperm  character. 
30 


174  PLANT   STRUCTURES 

95.  The  plant  body. — The  great  body  of  the  plant,  often 
forming  a  large  tree,  is  the  sporophyte ;  in  fact,  the 
gametophytes  are  not  visible  to  ordinary  observation.  It 
should  be  remembered  that  the  sporophyte  is  distinctly  a 
sexless  generation,  and  that  it  develops  no  sex  organs. 
This  great  sporophyte  body  is  elaborately  organized  for 
nutritive  work,  with  its  roots,  stems,  and  leaves.  These 
organs  are  very  complex  in  structure,  being  made  up  of 
various  tissue  systems  that  are  organized  for  special  kinds 
of  work.  The  leaves  are  the  most  variable  organs,  being 
differentiated  into  three  distinct  kinds — (1)  foliage  leaves, 
(2)  scales,  and  (3)  sporophylls. 

96.  Sporophylls. — The  sporophylls  are  leaves  set  apart  to 
produce  sporangia,  and  in  the  pine  they  are  arranged  in 
a  strobilus,  as  in  the  Horsetails  and  Club-mosses.  As 
the  group  is  heterosporous,  however,  there  are  two  kinds 
of  sporophylls  and  two  kinds  of  strobili.  One  kind  of 
strobilus  is  made  up  of  megasporophylls  bearing  mega- 
sporangia  ;  the  other  is  made  up  of  microsporophylls  bear- 
ing microsporangia.  These  strobili  are  often  spoken  of  as 
the  "  flowers  "  of  the  pine,  but  if  these  are  flowers,  so  are 
the  strobili  of  Horsetails  and  Club-mosses. 

97.  Microsporophylls. — In  the  pines  the  strobilus  com- 
posed of  microsporophylls  is  comparatively  small  (Figs. 
145,  d,  164).  Each  sporophyll  is  like  a  scale  leaf,  is  nar- 
rowed at  the  base,  and  upon  the  lower  surface  are  borne 
two  prominent  sporangia,  which  of  course  are  microspo- 
rangia, and  contain  microspores  (Fig.  146). 

These  structures  of  Seed-plants  all  received  names 
before  they  were  identified  with  the  corresponding  struc- 
tures of  the  lower  groups.  The  microsporophyll  was  called  a 
stamen,  the  microsporangia  pollen-sacs,  and  the  microspores 
pollen  grains,  or  simply  pollen.  These  names  are  still  very 
convenient  to  use  in  connection  with  the  Spermatophytes, 
but  it  should  be  remembered  that  they  are  simply  other 
names  for  structures  found  in  the  lower  groups. 


Fig.  145.  Mnus  Laricio,  showing  tip  of  branch  bearing  needle-leaves,  scale-leaves, 
and  cones  (strobili):  a,  very  young  carpellate  cones,  at  time  of  pollination,  borne 
at  tip  of  the  young  shoot  upon  which'  new  leaves  are  appearing;  b,  carpellate  cones 
one  year  old;  c,  carpellate  cones  two  years  old,  the  scales  spreading  and  shedding 
the  seeds;  d,  young  shoot  bearing  a  cluster  of  staminate  cones.— Caldwell. 


176 


PLANT   STRUCTURES 


The  strobilus  composed  of  microsporophylls  may  be 
called  the  staminate  strobilus — that  is,  one  composed  of 
stamens ;  it  is  often  called  the  staminate  cone,  "  cone " 
being  the  English  translation  of  the  word  "strobilus." 
Frequently  the  staminate  cone  is  spoken  of  as  the  "male 
cone,"  as  it  was  once  supposed  that  the  stamen  is  the 


Fig.  146.  Staminate  cone  (strobilus)  of  pine  (Pinus):  A,  section  of  cone,  showing 
microsporophylls  (stamens)  bearing  microsporangia;  B,  longitudinal  section  of  a 
single  stamen,  showing  the  large  sporangium  beneath  ;  C,  cross-section  of  a  sta- 
men, showing  the  two  sporangia;  D,  a  single  microspore  (pollen  grain)  much  en- 
larged, showing  the  two  wings,  and  a  male  gametophyte  of  two  cells,  the  lower 
and  larger  (wall  cell)  developing  the  pollen  tube,  the  upper  and  smaller  (genera- 
tive cell)  giving  rise  to  the  sperms.— After  Strasburgeb. 

male  organ.  This  name  should,  of  course,  be  abandoned, 
as  the  stamen  is  now  known  to  be  a  microsporophyll,  which 
is  an  organ  produced  by  the  sporophyte,  which  never  pro- 
duces sex  organs.  It  should  be  borne  distinctly  in  mind 
that  the  stamen  is  not  a  sex  organ,  for  the  literature  of 
botany  is  full  of  this  old  assumption,  and  the  beginner  is  in 


SPEKMATOPHYTES :  UYMNOKPERMS 


177 


danger  of  becoming  confused  and  of  forgetting  that  pollen 
grains  are  asexual  spores. 

98.  Megasporophylls—  The  strobili  composed  of  mega- 
sporophylls  become  much  larger  than  the  others,  forming 


Fig.  147.  Firms  sylvestris,  showing  mature  cone  partly  sectioned,  and  showing  car- 
pels (sq,  sq1,  sq*)  with  seeds  in  their  axils  (g),  in  which  the  embryos  (em)  may  be 
distinguished;  A,  a  young  carpel  with  two  megasporangia;  B,  an  old  carpel  with 
mature  seeds  (ch),  the  micropyle  being  below  (M). — After  Bessey. 


the  well-known  cones  so  characteristic  of  pines  and  their 
allies  (Figs.  145,  «,  b,  c,  163).  Each  sporophyll  is  some- 
what leaf -like,  and  at  its  base  upon  the  upper  side  are  two 
megasporangia  (Fig.  147).  It  is  these  sporangia  which  are 
peculiar  in  each  producing  and  retaining  a  solitary  large 
megaspore.     This  megaspore  resembles  a  sac-like  cavity  in 


178 


PLANT   STRUCTURES 


the  body  of  the  sporangium  (Fig.  148,  d),  and  was  at  first 
not  recognized  as  being  a  spore. 

These  structures  had  also  received  names  before  they 
were  identified  with  the  corresponding  structures  of  the 
lower  groups.  The  megasporophyll  was  called  a  carpel, 
the  megasporangia  ovules,  and  the  megaspore  an  embryo- 
sac,  because  the  young  embryo  was  observed  to  develop 
within  it  (Fig.  147,  em). 

The  strobilus  of  megasporophylls,  therefore,  may  be 
called  the  carpellate  strobilus  or  carpellate  cone.  As  the 
carpel  enters  into  the  organization  of  a  structure  known  as 
the  pistil,  to  be  described  later,  the  cone  is  often  called 
the  pistillate  cone.  As  the  staminate  cone  is  sometimes 
wrongly  called  a  "male  cone,"  so  the  carpellate  cone  is 
wrongly  called  a  "female  cone,"  the 
old  idea  being  that  the  carpel  with 
its  ovules  represented  the  female  sex 
organ. 

The  structure  of  the  megaspo- 
rangium,  or  ovule,  must  be  known. 
The  main  body  is  the  nucellus  (Figs. 
148,  c,  149,  nc) ;  this  sends  out  from 
near  its  base  an  outer  membrane 
{integument)  which  is  distinct  above 
(Figs.  148  b,  149  i),  covering  the  main 
part  of  the  nucellus  and  projecting 
beyond  its  apex  as  a  prominent  neck, 
the  passage  through  which  to  the  apex 
of  the  nucellus  is  called  the  micropyle 
("little  gate")  (Fig.  148,  a).  Cen- 
trally placed  within  the  body  of  the 
nucellus  is  the  conspicuous  cavity 
called  the  embryo-sac  (Fig.  148,  d), 
in  reality  the  retained  megaspore. 
The  relations  between  integument,  micropyle,  nucellus, 
and  embryo-sac  should  be  kept  clearly  in  mind.     In  the 


Fig.  148.  Diagram  of  the 
carpel  structures  of  pine, 
showing  the  heavy  scale 
(A)  which  bears  the 
ovule  (2?),  in  which  are 
seen  the  micropyle  (a), 
integument  (&),  nucellus 
(c),  embryo  sac  or  mega- 
spore (d). — Moorb. 


SPERMAT0PI1YTES :   GYMNOSPERMS 


179 


pine  the  micropyle  is  directed  downward,  toward  the  base 
of  the  sporophyll  (Figs.  147,  148). 

99.  Female  gametophyte. — The  female  gametophyte  is 
always  produced  by  the  germination  of  a  megaspore,  and 
therefore  it  should  be 
produced  by  the  so- 
called  embryo-sac  with- 
in the  ovule.  This  im- 
bedded megaspore  ger- 
minates, just  as  does 
the  megaspore  of  Se- 
laginetta  or  Isoetes,  by 
cell  division  becoming 
filled  with  a  compact 
mass  of  nutritive  tissue 
representing  the  ordi- 
nary cells  of  the  female 
prothallium  (Fig.  149, 
e).  This  prothallium 
naturally  does  not 
protrude  beyond  the 
boundary  of  the  mega- 
spore wall,  being  com- 
pletely surrounded  by 
the  tissues  of  the 
sporangium.  It  must 
be  evident  that  this 
gametophyte  is  abso- 
lutely dependent  upon 
the  sporophyte  for  its 
nutrition,  and  remains 
not  merely  attached  to 
it,  but  is  actually  im- 
bedded within  its  tis- 
sues like  an  internal  parasite.  So  conspicuous  a  tissue 
within  the  ovule,  as  well  as  in  the  seed  into  which  the 


Fig.  149.  Diagrammatic  section  through  ovule 
(megasporangium)  of  spruce  (Picea),  showing 
integument  (i),  nucellus  (nc),  endosperm  or 
female  gametophyte  (e)  which  fills  the  large 
megaspore  imbedded  in  the  nucellus,  two 
archegonia  («)  with  short  neck  (c)  and  venter 
containing  the  egg  (o),  and  position  of  ger- 
minating pollen  grains  or  microspores  (p) 
whose  tubes  (t)  penetrate  the  nucellus  tissue 
and  reach  the  archegonia.— After  Schimper. 


180  PLANT  STRUCTURES 

ovule  develops,  did  not  escape  early  attention,  and  it  was 
called  endosperm,  meaning  "within  the  seed."  The  endo- 
sperm of  Gymnosperms,  therefore,  is  the  female  gameto- 
phyte. 

At  the  margin  of  the  endosperm  nearest  the  micropyle 
regular  flask-shaped  archegonia  are  developed  (Fig.  149,  a), 
making  it  sure  that  the  endosperm  is  a  female  gameto- 
phyte.  It  is  evident  that  the  necks  of  these  archegonia 
(Fig.  149,  c)  are  shut  away  from  the  approach  of  sperms  by 
swimming,  and  that  some  new  method  of  approach  must  be 
developed. 

100.  Male  gametophyte. — The  microspores  are  developed 
in  the  sporangium  in  the  usual  tetrad  fashion,  and  are  pro- 
duced and  scattered  in  very  great  abundance.  It  will  be 
remembered  that  the  male  gametophyte  developed  by  the 
microspore  of  Selaginella  is  contained  entirely  within  the 
spore,  and  consists  of  a  single  ordinary  prothallial  cell 
and  one  antheridium  (see  §  89).  In  the  pine  it  is  no  bet- 
ter developed.  One  or  two  small  cells  appear,  which  may 
be  regarded  as  representing  prothallial  cells,  while  the  rest 
of  the  gametophyte  seems  to  be  a  single  antheridium  (Fig. 
146,  D).  At  first  this  antheridium  seems  to  consist  of  a 
large  cell  called  the  wall  cell,  and  a  small  one  called  the 
generative  cell.  Sooner  or  later  the  generative  cell  divides 
and  forms  two  small  cells,  one  of  which  divides  again  and 
forms  two  cells  called  male  cells,  which  seem  to  represent 
the  sperm  mother  cells  of  lower  plants.  The  three  active 
cells  of  the  completed  antheridium,  therefore,  are  the  wall 
cell,  with  a  prominent  nucleus,  and  two  small  male  cells 
which  are  free  in  the  large  wall  cell. 

These  sperm  mother  cells  (male  cells)  do  not  form 
sperms  within  them,  as  there  is  no  water  connection  be- 
tween them  and  the  archegonia,  and  a  new  method  of 
transfer  is  provided.  This  is  done  by  the  wall  cell,  which 
develops  a  tube,  known  as  the  pollen-tube.  Into  this  tube 
the  male  cells  enter,  and  as  it  penetrates  among  the  cells 


SPEEMATOPHYTES :  GYMNOSPEEMS 


181 


which  shut  off  the  archegonia  it  carries  the   male  cells 
along,  and  so  they  are  brought  to  the  archegonia  (Fig.  150). 


Fig.  150.  Tip  of  pollen  tube  of  pine, 
showing  the  two  male  cells  (.4,  B), 
two  nuclei  ( C)  which  accompany 
them,  and  the  numerous  food 
granules  (Z>) :  the  tip  of  the  tube 
is  just  about  to  enter  the  neck  of 
the  archegonium.— Caldwell. 


Fig.  151.  Pollen  tube  passing  through  the 
neck  of  an  archegonium  of  spruce  (Picea), 
and  containing  near  its  tip  the  two  male 
nuclei,  which  are  to  be  discharged  into  the 
egg  whose  cytoplasm  the  tube  is  just  en- 
tering.— After  Strasburger. 


101.  Fertilization. — Before  fertilization  can  take  place 
the  pollen-grains  (microspores)  must  be  brought  as  near  as 
possible  to  the  female  gametophyte  with  its  archegonia. 
The  spores  are  formed  in  very  great  abundance,  are  dry 
and  powdery,  and  are  scattered  far  and  wide  by  the  wind. 
In  the  pines  and  their  allies  the  pollen-grains  are  winged 
(Fig.  146,  D),  so  that  they  are  well  organized  for  wind  dis- 
tribution. This  transfer  of  pollen  is  called  pollination,  and 
those  plants  that  use  the  wind  as  an  agent  of  transfer  are 
said  to  be  anemophilotis,  or  "wind-loving." 

The  pollen  must  reach  the  ovule,  and  to  insure  this  it 
must  fall  like  rain.  To  aid  in  catching  the  falling  pollen 
the  scale-like  carpels  of  the  cone  spread  apart,  the  pollen 
grains  slide  down  their  sloping  surfaces  and  collect  in  a 


182 


PLANT  STRUCTURES 


little  drift  at  the  bottom  of  each  carpel,  where  the  ovules 
are  found  (Fig.  147,  A,  B).  The  flaring  lips  of  the  micro- 
pyle  roll  inward  and  outward  as  they  are  dry  or  moist,  and 
by  this  motion  some  of  the  pollen-grains  are  caught  and 
pressed  down  upon  the  apex  of  the  nucellus. 

In  this  position  the  pollen-tube  develops,  crowds  its 
way  among  the  cells  of  the  nucellus,  reaches  the  wall  of 
the  embryo-sac,  and  penetrating  that,  reaches  the  necks 
of  the  archegonia  (Fig.  149,  p,  t)  ;  crowding  into  them 
(Fig.  151),  the  tip  of  the  tube  opens,  the  male  cells  are 


an 


■ 


mmsm 


Fig.  152.  Fertilization  in  spruce  (Picea):  B  is  an  egg,  in  the  tip  of  which  a  pollen 
tube  (p)  has  entered  and  has  discharged  into  the  cytoplasm  a  male  nucleus  (m), 
which  is  to  unite  with  the  egg  (female)  nucleus  (on);  0,  a  later  stage  in  which  the 
two  nuclei  are  uniting. — After  Schimper. 


discharged,  one  male  cell  fuses  with  the  egg  (Fig.  152), 
and  fertilization  is  accomplished,  an  oospore  being  formed 
in  the  venter  of  the  archegonium. 

It  will  be  noticed  that  the  cell  which  acts  as  a  male 
gamete  is  really  the  sperm  mother  cell,  which  does  not 
organize  a  sperm  in  the  absence  of  a  water  connection. 
This  peculiar  method  of  transferring  the  male  cells  by 
means  of  a  special  tube  developed  by  the  antheridium  is 


SPEEMATOPHYTES:  GYMNOSPEKMS 


183 


called  siphonogamy,  which  means  "  sexual  reproduction  by 
means  of  a  tube."  So  important  is  this  character  among 
Spermatophytes  that  some  have  proposed  to  call  the  group 
Siphonogams. 

102.  Development  of  the  embryo. — The  oospore  when 
formed  lies  at  the  surface  of  the  endosperm  (female  gameto- 
phyte)  nearest  to  the  micropyle.  As  the  endosperm  is  to 
supply  nourishment  to  the  em- 
bryo, this  position  is  not  the 
most  favorable.  Therefore,  as 
in  Selaginella,  the  oospore  first 
develops  a  suspensor,  which  in 
pine  and  its  allies  becomes  very 
long  and  often  tortuous  (Fig. 
153,  A,  s).  At  the  tip  of  the 
suspensor  the  cell  or  cells  (em- 
bryo cells)  which  are  to  develop 
the  embryo  are  carried  (Fig.  153, 
A,  ha),  and  thus  become  deeply 
buried,  about  centrally  placed, 
in  the  endosperm. 

Several  suspensors  may  start 
from  as  many  archegonia  in  the 
same  ovule,  and  several  embryos 
may  begin  to  develop,  but  as  a 
rule  only  one  survives,  and  the 
solitary  completed  embryo  (Fig. 
153,  B)  lies  centrally  imbedded 

in  the  endosperm  (Fig.  153a).  The  development  of  more 
than  one  embryo  in  a  megasporangium  (ovule)  is  called 
polyembryony,  a  phenomenon  natural  to  Gymnosperms  with 
their  several  archegonia  upon  a  single  gametophyte. 

103.  The  seed. — While  the  embryo  is  developing  some 
important  changes  are  taking  place  in  the  ovule  outside 
of  the  endosperm.  The  most  noteworthy  is  the  develop- 
ment of  a  special  tissue  that  forms  a  hard  bony  covering, 


Fig.  153.  Embryos  of  pine:  A, 
very  young  embryos  (ka)  at  the 
tips  of  long  and  contorted  sus- 
pensors (s) ;  B,  older  embryo, 
showing  attachment  to  suspen- 
sor («),  the  extensive  root  sheath 
(tvh),  root  tip  (ws),  stem  tip 
(v),  and  cotyledons  (c).— After 
Strasburger. 


184 


PLANT  STRUCTURES 


known  as  the  seed  coat,  or  testa  (Fig.  153a).  The  devel- 
opment of  this  testa  hermetically  seals  the  structures  with- 
in, further  development  and  activity 
are  checked,  and  the  living  cells  pass 
into  the  resting  condition.  This  pro- 
tected structure  with  its  dormant  cells 
is  the  seed. 

In  a  certain  sense  the  seed  is  a  transformed  ovule  (mega- 
sporangium),  but  this  is  true  only  as  to  its  outer  configura- 


Fig.  153a.    Pine  seed. 


tZrZjT' 


Fig.  154. 


Pine  seedlings,  showing  the  long  hypocotyl  and  the  numerous  cotyledons, 
with  the  old  seed  case  still  attached.— After  Atkinson. 


SPERMATOPHYTES:  GYMNOSPERMS 


185 


tion.  If  the  internal  structures  be  considered  it  is  much 
more.  It  is  made  up  of  structures  belonging  to  three  gen- 
erations, as  follows  :  (1)  The  old  sporophyte  is  represented 
by  seed  coat  and  nucellus,  (2)  the  endosperm  is  a  gameto- 
phyte,  while  (3)  the  embryo  is  a  young  sporophyte.  It  can 
hardly  be  said  that  the  seed  is  simple  in  structure,  or  that 
any  real  conception  of  it  can  be  obtained  without  approach- 
ing it  by  way  of  the  lower  groups. 

The  organization  of  the  seed  checks  the  growth  of  the 
embryo,  and  this  development  within  the  seed  is  known  as 


Fig.  155.    A  cycad,  showing  the  palm-like  habit,  with  much  branched  leaves  and 
scaly  stem.— From  "  Plant  Relations." 


the  intra-seminal  development.  In  this  condition  the  em- 
bryo may  continue  for  a  very  long  time,  and  it  is  a  ques- 
tion whether  it  is  death  or  suspended  animation.  Is  a  seed 
alive  ?  is  not  an  easy  question  to  answer,  for  it  may  be  kept 
in  a  dried-out  condition  for  years,  and  then  when  placed 
in  suitable  conditions  awaken  and  put  forth  a  living  plant. 


SPERMATOPHYTES :  GYMNOSPERMS 


187 


This  "  awakening  "  of  the  seed  is  spoken  of  as  its  "  ger- 
mination," but  this  must  not  be  confused  with  the  germi- 
nation of  a  spore,  which  is  real  germination.  In  the  case 
of  the  seed  an  oospore  has  germinated  and  formed  an  embryo, 
which  stops  growing  for  a  time,  and  then  resumes  it.  This 
resumption   of  growth  is  not  germination,  but  is  what 


Fig.  157.    Tip  of  pollen  tube  of  Cycas  rexoluta.  containing  the  two  spiral,  multiciliate 
sperms. — After  Ikeno. 

happens  when  a  seed  is  said  to  "germinate."  This  second 
period  of  development  is  known  as  the  extra-seminal,  for  it 
is  inaugurated  by  the  escape  of  the  sporophyte  from  the 
seed  (Fig.  154). 

104.    The  great  groups  of  Gymnosperms. — There  are  at 
least  four  living  groups  of  Gymnosperms,  and  two  or  three 


Fig.  158.    A  pine  {Finns)  showing  the  central  shaft  and  also  the  bunching  of  the 
needle  leaves  toward  the  tips  of  the  branches.— From  "  Plant  Relations." 


SPERMATOPHYTES :  GYMNOSPERMS 


189 


extinct  ones.     The  groups  differ  so  widely  from  one  an- 
other in  habit  as  to  show  that  Gymnosperms  can  be  very 
much  diversified.    They  are  all  woody  forms,  but  they  may 
be  trailing  or  straggling 
shrubs,  gigantic  trees,  or 
high-climbing  vines ;  and 
their  leaves  may  be  nee- 
dle-like, broad,  or  "fern- 
like."   For  our  purpose  it 
will  be  only  necessary  to 
define  the  two  most  prom- 
inent groups. 

105.  Cycads.  —  Cycads 
are  tropical,  fern  -  like 
forms,  with  large  branched 
(compound)  leaves.  The 
stem  is  either  a  columnar 
shaft  crowned  with  a  ro- 
sette of  great  branching 
leaves,  with  the  general 
habit  of  tree-ferns  and 
palms  (Figs.  155,  156)  ; 
or  they  are  like  great  tu- 
bers, crowned  in  the  same 
way.  In  ancient  times 
(the  Mesozoic)  they  were 
very  abundant,  forming 
a  conspicuous  feature  of 
the  vegetation,  but  now 
they  are  represented  only 
by  about  eighty  forms 
scattered  through  both 
the  oriental  and  occiden- 
tal tropics. 

They    are    very   fern- 
like in  structure  as  well 
31 


Fig.  159.  The  giant  redwood  {Sequoia  gi- 
gantea)  of  California :  the  relative  size 
is  indicated  by  the  figure  of  a  man  stand- 
ing at  the  right.— After  Williamson. 


190 


PLANT  STRUCTURES 


as  in  appearance,  but  they  produce  seeds  and  must  be 
associated  with  Spermatophytes,  and  as  the  seed  is  ex- 
posed they  are  Gymnosperms.     A  discovery  has  been  made 

recently  that  strikingly 
JIkK  emphasizes  their  fern- 
like structure.  In  fer- 
tilization a  pollen-tube 
develops,  as  described 
for  pine  and  its  allies, 
but  the  male  cells 
(sperm  mother  -  cells) 
which  it  contains  or- 
ganize sperms,  and 
these  sperms  are  of 
the  coiled  multiciliate 
type  (Fig.  157)  charac- 
teristic of  all  the  Pter- 
idophytes  except  Club- 
mosses.  This  associa- 
tion of  the  old  ciliated 
sperm  habit  with  the 
new  pollen-tube  habit 
is  a  very  interesting  in- 
termediate or  transition 
condition.  It  should  be 
said  that  these  sperms 
have  been  actually  found 
in  but  few  species  of 
the  Cycads,  but  there 
are  reasons  for  suppos- 
ing that  they  may  be 
found  in  all.  Another 
one  of  the  Gymnosperm 

Fig.    160.     An   araucarian   pine   (Araucaria),  ^  * 

showing  the  central  shaft,  and  the  regular  groups,  represented  tO- 

cycles  of  branches  spreading  in  every  direc-  ^ay    only    by    the    COm- 
tion  and  bearing  numerous  small  leaves.—  .   .  ,  .  , 

From  "  Plant  Relations."  monly  cultivated  maid- 


SPERM  ATOPHYTES:  GYMMOSPEKMS 


191 


enhair  tree  (Gingko),  with  broad  dichotomously  veined 
leaves,  also  develops  multiciliate  sperms. 

The  testa  of  the  seed,  instead  of  being  entirely  hard  as 
described  for  pine  and  its  allies,  develops  in  two  layers,  the 
inner  hard  and  bony,  and  the  outer  pulpy,  making  the  ripe 
fruit  resemble  a  plum. 

106.  Conifers. — This  is  the  great  modern  Gymnosperm 
group,  and  is  characteristic  of  the  temperate  regions,  where 
it  forms  great  forests.  Some  of  the  forms  are  widely  dis- 
tributed, as  the  great  genus  of  pines  (Finns)  (Fig.  158), 
while  some  are  now  very  much  restricted,  although  for- 
merly very  widely  distributed,  as  the  gigantic  redwoods 
(Sequoia)  of  the  Pacific  slope  (Fig.  159).  The  habit  of 
the  body  is  quite  charac- 
teristic, a  central  shaft 
extending  continuously  to 
the  very  top,  while  the 
lateral  branches  spread 
horizontally,  with  dimin- 
ishing length  to  the  top, 
forming  a  conical  outline 
(Figs.  160,  162).  This 
habit  of  firs,  pines,  etc., 
gives  them  an  appearance 
very  distinct  from  that  of 
other  trees. 

Another  peculiar  fea- 
ture is  furnished  by  the 
characteristic  "needle- 
leaves,"  which  seem  to  be 

poorly  adapted  for  foliage.  These  leaves  have  small  spread 
of  surface  and  very  heavy  protecting  walls,  and  show 
adaptation  for  enduring  hard  conditions  (Fig.  161).  As 
they  have  no  regular  period  of  falling,  the  trees  are  always 
clothed  with  them,  and  have  been  called  "  evergreens." 
There  are  some  notable  exceptions  to  this,  however,  as  in 


Fig.  161. — Cross-section  of  a  needle-leaf  of 
pine,  showing  epidermis  (e)  in  which 
there  are  sunken  stomata  (sp),  heavy- 
walled  hypodermal  tissue  («s)  which 
gives  rigidity,  the  mesophyll  region  (p) 
in  which  a  few  resin-ducts  (h)  are  seen, 
and  the  central  region  {stele)  in  which 
two  vascular  bundles  are  developed.— 
After  Sachs. 


Fig.  162.  A  larch  (Larix),  showing  the  continuous  central  shaft  and  horizontal 
branches,  the  general  outline  being  distinctly  conical.  The  larch  is  peculiar 
among  Conifers  in  periodically  shedding  its  leaves.— From  "  Plant  Relations." 


SPERMATOPHYTES:  GYMNOSPERMS 


193 


the  case  of  the  common  larch  or  tamarack,  which  sheds 
its  leaves  every  season  (Fig.  162).  There  are  Conifers, 
also,  which  do  not  produce  needle-leaves,  as  in  the  com- 
mon arbor-vitae,  whose  leaves  consist  of  small  closely-over- 
lapping scale-like  bodies 
(Fig.  163). 

The  two  types  of  leaf 
arrangement  may  also  be 
noted.  In  most  Conifers 
the  leaves  are  arranged 
along  the  stem  in  spiral 
fashion,  no  two  leaves 
being  at  the  same  level. 
This  is  known  as  the  spi- 
ral or  alternate  arrange- 
ment. In  other  forms,  as 
the  cypresses,  the  leaves 
are  in  cycles,  as  was  men- 
tioned in  connection  with 
the  Horsetails,  the  ar- 
rangement being  known 
as  the  cyclic  or  whorled. 

The  character  which 
gives  name  to  the  group 
is  the  "cone" — that  is, 
the  prominent  carpellate 
cone    which    becomes    so 


Fig.  163.  Arbor-vitae  (Thuja),  showing  a 
branch  with  scaly  overlapping  leaves, 
and  some  carpellate  cones  (strobili).— 
After  Eichler. 


conspicuous  in  connec- 
tion with  the  ripening  of 
the  seeds.  These  cones 
generally   ripen    dry  and 

hard  (Figs.  145,  147,  163),  but  sometimes,  as  in  junipers, 
they  become  pulpy  (Fig.  164),  the  whole  cone  forming  the 
so-called  "berry." 

There  are  two  great  groups  of   Conifers.     One,  repre- 
sented by  the  pines,  has   true   cones  which  conceal  the 


194  PLANT  STKUCTUKES 

ovules,  and  the  seeds  ripen  dry.  The  other,  represented 
by  the  yews,  has  exposed  ovules,  and  the  seed  either  ripens 
fleshy  or  has  a  fleshy  investment. 


Fig.  164.  The  common  juniper  {Juniperus  communis);  the  branch  to  the  left  bearing 
staminate  strobili;  that  to  the  right  bearing  staminate  strobili  above  and  carpel- 
late  strobili  below,  which  latter  have  matured  into  the  fleshy,  berry-like  fruit, 
— After  Berg  and  Schmidt. 


CHAPTER   XII 

SPERMATOPHYTES :  ANGIOSPERMS 

107.  Summary  of  Gymnosperms. — Before  beginning  An- 
giosperms  it  is  well  to  state  clearly  the  characters  of  Gym- 
nosperms which  have  set  them  apart  as  a  distinct  group  of 
Sperniatophytes,  and  which  serve  to  contrast  them  with 
Angiosperms. 

(1)  The  microspore  (pollen-grain)  by  wind-pollination 
is  brought  into  contact  with  the  megasporangium  (ovule), 
and  there  develops  the  pollen-tube,  which  penetrates  the 
nucellus.  This  contact  between  pollen  and  ovule  implies 
an  exposed  or  naked  ovule  and  hence  seed,  and  therefore 
the  name  "  Gymnosperm." 

(2)  The  female  gametophyte  (endosperm)  is  well  organ- 
ized before  fertilization. 

(3)  The  female  gametophyte  produces  archegonia. 

108.  General  characters  of  Angiosperms. — This  is  the  great- 
est group  of  plants,  both  in  numbers  and  importance,  being 
estimated  to  contain  about  100,000  species,  and  forming 
the  most  conspicuous  part  of  the  vegetation  of  the  earth. 
It  is  essentially  a  modern  group,  replacing  the  Gymnosperms 
which  were  formerly  the  dominant  Seed-plants,  and  in  the 
variety  of  their  display  exceeding  all  other  groups.  The 
name  of  the  group  is  suggested  by  the  fact  that  the  seeds 
are  inclosed  in  a  seed  case,  in  contrast  with  the  exposed 
seeds  of  the  Gymnosperms. 

These  are  also  the  true  flowering  plants,  and  the  ap- 
pearance  of  true   flowers   means   the  development  of  an 

195 


196 


PLANT  STRUCTURES 


elaborate  symbiotic  relation  between  flowers  and  insects, 
through  which  pollination  is  secured.  In  Angiosperms, 
therefore,  the  wind  is  abandoned  as  an  agent  of  pollen 
transfer  and  insects  are  used ;  and  in  passing  from  Gym- 
nosperms  to  Angiosperms  one  passes  from  anemophilous  to 
entomophilous  ("insect-loving")  plants.  This  does  not 
mean  that  all  Angiosperms  are  entomophilous,  for  some  are 
still  wind-pollinated,  but  that  the  group  is  prevailingly  ento- 
mophilous. This  fact,  more  than  anything  else,  has  re- 
sulted in  a  vast  variety  in  the  structure  of  flowers,  so  char- 
acteristic of  the  group. 

109.  The  plant  body. — This  of  course  is  a  sporophyte, 
the  gametophytes  being  minute  and  concealed,  as  in  Gym- 
nosperms.  The  sporophyte  represents  the  greatest  possible 
variety  in  habit,  size,  and  duration,  from  minute  floating 
forms  to  gigantic  trees  ;  herbs,  shrubs,  trees  ;  erect,  pros- 
trate, climbing  ;  aquatic,  terrestrial,  epiphytic  ;  from  a  few 
days  to  centuries  in  duration. 

Koots,  stems,  and  leaves  are  more  elaborate  and  vari- 
ously organized  for  work  than  in  other  groups,  and  the 
whole  structure  represents  the  high- 
est organization  the  plant  body  has 
attained.  As  in  the  Gymnosperms, 
the  leaf  is  the  most  variously  used 
organ,  showing  at  least  four  distinct 
modifications  :  (1)  foliage  leaves,  (2) 
scales,  (3)  sporophylls,  and  (4)  floral 
leaves.  The  first  three  are  present 
in  Gymnosperms,  and  even  in  Pteri- 
dophytes,  but  floral  leaves  are  pecul- 
iar to  Angiosperms,  making  the  true 
flower,  and  being  associated  with  en- 
tomophily. 

110.  Microsporophylls. — The  micro- 
sporophyll  of  Angiosperms  is  more 
definitely  known  as  a  "  stamen  "  than 


Fig.  165.  Stamens  of  hen- 
bane (Hyoscyamue) :  A, 
front  view,  showing  fila- 
ment (/)  and  anther  {p)\ 
B,  back  view,  showing 
the  connective  (c)  be- 
tween the  pollen-sacs. 
—After  Schimper. 


SPERMATOPHYTES :  ANGIOSPERMS 


197 


that  of  Gymnosperms,  and  has  lost  any  semblance  to  a  leaf. 
It  consists  of  a  stalk-like  portion,  the  filament;  and  a 
sporangia  -  bearing  portion,  the   anther  (Figs.   165,  167«). 


Fig.  166.  Cross-section  of  anther  of  thorn  apple  (Datura),  showing  the  four  imbedded 
sporangia  (a,  p)  containing  microspores;  the  pair  on  each  side  will  merge  and 
dehisce  along  the  depression  between  them  for  the  discharge  of  pollen. — After 
Frank. 

The  filament  may  be  long  or  short,  slender  or  broad,  or 
variously  modified,  or  even  wanting.  The  anther  is  simply 
the  region  of  the  sporophyll  which  bears  sporangia,  and  is 


Fig.  167.  Diagrammatic  cross-sections  of  anthers:  A,  younger  stage,  showing  the 
four  imbedded  sporangia,  the  contents  of  two  removed,  but  the  other  two  con- 
taining pollen  mother  cells  (p?n)  surrounded  by  the  tapetum  «);  B,  an  older  stage, 
in  which  the  microspores  (pollen  grains)  are  mature,  and  the  pair  of  sporangia  on 
each  side  are  merging  together  to  form  a  single  pollen-sac  with  longitudinal 
dehiscence.— After  Baillon  and  Luerssen. 

therefore  a  composite  of  sporophyll  and  sporangia  and  is 
often  of  uncertain  limitation.  Such  a  term  is  convenient, 
but  is  not  exact  or  scientific. 


198 


PLANT  STEOCTUEES 


If  a  young  anther  be  sectioned  transversely  four  sporan- 
gia will  be  found  imbedded  beneath  the  epidermis,  a  pair 
on  each  side  of  the  axis  (Figs.  166, 167).  When  they  reach 
maturity,  the  paired  sporangia  on  each  side  usually  merge  to- 
gether, forming  two  spore-containing  cavities  (Fig.  167,  B). 
These  are  generally  called  "  pollen-sacs,"  and  each  anther  is 
said  to  consist  of  two  pollen-sacs,  although  each  sac  is  made 
up  of  two  merged  sporangia,  and  is  not  the  equivalent  of  the 
pollen-sac  in  Gymnosperms,  which  is  a  single  sporangium. 


Fig.  167a,  Various  forms  of  stamens:  A,  from  Solanum,  showing  dehiscence  by 
terminal  pores;  B,  from  Arbutus,  showing  anthers  with  terminal  pores  and 
"horns";  C,  from  Berberis;  B,  from  Atherosperma,  showing  dehiscence  by 
uplifted  valves;  E,  from  Aquilegia,  showing  longitudinal  dehiscence;  F,  from 
Popowia.  showing  pollen-sacs  near  the  middle  of  the  stamen.— After  Enuler 
and  Prantl. 


SPERMATOPHYTES:   ANGIOSPEKMS 


199 


The  opening  of  the  pollen-sac  to  discharge  its  pollen- 
grains  (microspores)  is  called  dehiscence,  which  means  "a 
splitting  open,"  and  the  methods  of 
dehiscence  are  various  (Fig.  167a). 
By  far  the  most  common  method 
is  for  the  wall  of  each  sac  to  split 
lengthwise  (Fig.  1G8),  which  is 
called  longitudinal  dehiscence;  an- 
other is  for  each  sac  to  open  by  a 
terminal  pore  (Fig.  167«),  in  which 
case  it  may  be  prolonged  above  into 
a  tube. 

111.  Megasporophylls.  —  These 
are  the  so-called  "  carpels  "  of  Seed- 
plants,  and  in  Angiosperms  they 
are  organized  in  various  ways,  but 
always  so  as  to  inclose  the  mega- 
sporangia  (ovules).    In  the  simplest 

cases  each  carpel  is  independent  (Fig.  169,  A),  and  is  dif- 
ferentiated into  three  regions :    (1)  a  hollow  bulbous  base, 

which  contains  the 


Fig.  168.  Cross  -  section  of 
anther  of  a  lily  (Butomus), 
showing  the  separating  walls 
between  the  members  of  each 
pair  of  sporangia  broken 
down  at  z,  forming  a  con- 
tinuous cavity  (pollen  sac) 
which  opens  by  a  longitudi- 
nal slit.— After  Sachs. 


the 


Fig.  169.  Types  of  pistils :  A,  three  simple  pistils 
(apocarpous),  each  showing  ovary  and  style  tipped 
with  stigma ;  B,  a  compound  pistil  (syncarpous), 
showing  ovary  (/),  separate  styles  (g),  and  stigmas 
(n) ;  C,  a  compound  pistil  (syncarpous),  showing 
ovary  (/),  single  style  (g),  and  stigma  (n).— After 
Berg  and  Schmidt. 


ovules  and  is 
real  seed  case, 
known  as  the 
ovary ;  (2)  sur- 
mounting this  is  a 
slender  more  or  less 
elongated  process, 
the  style;  and  (3) 
usually  at  or  near 
the  apex  of  the  style 
a  special  receptive 
surface  for  the  pol- 
len, the  stigma. 

In   other  cases 
several  carpels  to- 


200 


PLANT   STKUCTUKES 


gether  form  a  common  ovary,  while  the  styles  may  also 
combine  to  form  one  style  (Fig.  169,  C),  or  they  may  remain 
more  or  less  distinct  (Fig.  169,  B).  Such  an  ovary  may 
contain  a  single  chamber,  as  if  the  carpels  had  united  edge 
to  edge  (Fig.  170,  A)  ;  or  it  may  contain  as  many  chambers 
as  there  are  constituent  carpels  (Fig.  170,  B),  as  though 
each  carpel  had  formed  its  own  ovary  before  coalescence. 
In  ordinary  phrase  an  ovary  is  either  "one-celled"  or 
"  several-celled,"  but  as  the  word  "  cell "  has  a  very  differ- 
ent application,  the  ovary  chamber  had  better  be  called  a 
loculus,  meaning  "a  compartment."     Ovaries, 


Fig.  170.  Diagrammatic  sections  of  ovaries:  A,  cross-section  of  an  ovary  with  one 
loculus  and  three  carpels,  the  three  sets  of  ovules'  said  to  be  attached  to  the  wall 
(parietal);  B,  cross-section  of  an  ovary  with  three  loculi  and  three  carpels,  the 
ovules  being  in  the  center  (central) ;  C\  longitudinal  section  of  B,  showing  ovules 
attached  to  free  axis  ("  free  central  '1). — After  Schimpeb. 

therefore,  may  have  one  loculus  or  several  loculi.  Where 
there  are  several  loculi  each  one  usually  represents  a  con- 
stituent carpel  (Fig.  170,  B) ;  where  there  is  one  loculus 
the  ovary  may  comprise  one  carpel  (Fig.  169,  A),  or  several 
(Fig.  170,  A). 

There  is  a  very  convenient  but  not  a  scientific  word, 
which  stands  for  any  organization  of  the  ovary  and  the 
accompanying  parts,  and  that  is  pistil.  A  pistil  may  be 
one  carpel  (Fig.  169,  A),  or  it  may  be  several  carpels  or- 
ganized together  (Fig.  169,  B,  C),  the  former  case  being  a 
simple  pistil,  the  latter  a  compound  pistil.     In  other  words, 


SPERMATOPHYTES :  ANGIOSPEEMS 


201 


any  organization  of  carpels  which  ap- 
pears as  a  single  organ  with  one  ovary 
is  a  pistil. 

The  ovules  (megasporangia)  are 
developed  within  the  ovary  (Fig.  170) 
either  from  the  carpel  wall,  when  they 
are  foliar,  or  from  the  stem  axis  which 
ends  within  the  ovary,  when  they  are 
cauline  (see  §  89).  They  are  similar 
in  structure  to  those  of  Gymnosperms, 
with  integument  and  micropyle,  nu- 
cellus,  and  embryo -sac  (megaspore), 
except  that  there  are  often  two  integu- 
ments, an  outer  and  an  inner  (Fig. 
171). 

112.  The  male  gametophyte. — When  the  pollen-grain 
(microspore)  germinates  there  is  formed  within  it  the  sim- 
plest known  gametophyte  (Fig.   172).      No  trace  of  the 


Fig.  171.  A  diagrammatic 
section  of  an  ovule  of 
Angiosperms,  showing 
outer  integument  (ai), 
inner  integument  (ii), 
micropyle  (m),  nucellus 
(k),  and  embryo  sac  or 
megaspore  (em).— After 
Sacks. 


Fig.  172.  Germination  of  microspore  (pollen  grain)  in  duckweed  (Lemna):  A,  mature 
spore  with  its  nucleus;  B,  nucleus  of  spore  dividing;  C,  two  nuclei  resulting  from 
the  division;  D,  a  large  and  small  cell  following  the  nuclear  division,  forming  the 
two-celled  male  gametophyte;  E,  division  of  smaller  cell  (generative)  to  form  the 
two  male  cells;  F,  the  two  male  cells  completed  and  lying  near  the  large  tube 
nucleus. — Caldwell. 


202 


PLANT   STRUCTURES 


ordinary  nutritive  cells  of  the  gametophyte  remains,  and 
the  whole  structure  seems  to  represent  a  single  antherid- 
ium.  At  first  it  consists  of  two  cells,  the  large  wall  cell 
and  the  small  free  generative  cell  (Fig.  172,  D).     Later 

the  generative  cell  di- 
vides (Fig.  172,  E), 
either  while  in  the 
pollen -grain  or  after 
entrance  into  the  pol- 
len-tube, and  two  male 
cells  (sperm  mother- 
cells)  are  formed  (Fig. 
172,  F),  which  do  not 
organize  sperms,  but 
which  function  direct- 
ly as  gametes. 

When  pollination 
occurs,  and  the  pollen 
has  been  transferred 
from  the  pollen-sacs 
to  the  stigma,  it  is  de- 
tained by  the  minute 
papillae  of  the  stig- 
matic  surface,  which 
also  excretes  a  sweet- 
ish sticky  fluid.  This 
fluid  is  a  nutrient  so- 
lution for  the  micro- 
spores, which  begin  to 
put  out  their  tubes. 
A  pollen-tube  pene- 
trates through  the 
stigmatic  surface,  en- 
ters among  the  tissues 
of  the  style,  which  is  sometimes  very  long,  slowly  or  rap- 
idly traverses  the  length  of  the  style  supplied  with  food  by 


Fig.  173.  Diagram  of  a  longitudinal  section  through 
a  carpel,  to  illustrate  fertilization  with  all  parts 
in  place  :  s,  stigma  ;  g,  style  ;  o,  ovary  ;  ai,  ii, 
outer  and  inner  integuments;  n,  base  of  nucel- 
lus  ;  /,  funiculus  ;  b,  antipodal  cells  ;  c,  endo- 
sperm nucleus;  *,  egg  and  one  synergid;  p,  pol- 
len-tube, having  grown  from  stigma  and  passed 
through  the  micropyle  (m)  to  the  egg. — After 
Lubussen. 


SPERMATOPHYTES  :   ANGIOSPEKMS 


203 


its  cells  but  not  penetrating  them,  enters  the  cavity  of  the 
ovary,  passes  through  the  micropyle  of  an  ovule,  penetrates 
the  tissues  of  the  nucellus  (if  any),  and  finally  reaches  and 
pierces  the  wall  of  the  embryo-sac,  within  which  is  the  egg 
awaiting  fertilization  (Fig.  173). 

This  remarkable  ability  of  the  pollen-tube  to  make  its 
way  through  so  much  tissue,  directly  to  the  micropyle  of 
an  inclosed  ovule,  can  only  be  explained  by  supposing  that 
it  is  under  the  guidance  of  some  strong  attraction. 

113.  The  female  gametophyte. — The  megaspore  (embryo- 
sac)  occupies  the  same  position  in  the  ovule  as  in  Gymno- 
sperms,  but  its  germination  is  remarkably  modified.  The 
development  of  the  female  gametophyte,  the  act  of  f  ertil- 


Fig.  174.  Lilium  PMladelphicum :  to  the  left  a  young  megasporangium  (ovule), 
showing  integuments  (  C),  nucellus  {A),  and  megaspore  (B)  containing  a  large  nu- 
cleus. To  the  right  a  megaspore  whose  nucleus  is  undergoing  the  first  division 
in  the  formation  of  the  gametophyte.— Caldwell. 


ization,  and  the  development  of  endosperm  are  the  three 
subjects  to  be  considered.  If  fertilization  is  not  accom- 
plished the  endosperm  is  usually  not  developed. 

Development— The    megaspore    nucleus    divides   (Fig. 
174),  and  one  nucleus  passes  to  each  end  of  the  embryo- 


204 


PLANT  STKUCTUEES 


sac  (Fig.  175,  at  left).  Each  of  these  nuclei  divide  (Fig. 
175,  at  right),  and  two  nuclei  appear  at  each  end  of  the 
sac  (Fig.  175,  at  middle).     Each  of  the  four  nuclei  divide 


Fig.  175.  Lilium  Philadelphicum :  to  the  left  is  an  embryo-sac  with  a  gametophyte 
nucleus  in  each  end;  to  the  right  these  two  nuclei  are  dividing  to  form  the  two 
nuclei  shown  in  each  end  of  the  sac  in  the  middle  figure.— Caldwell. 

(Fig.  176,  at  left),  and  four  nuclei  appear  at  each  end  (Fig. 
176,  at  middle).  When  eight  nuclei  have  appeared,  nuclear 
division  stops.  Then  a  remarkable  phenomenon  occurs. 
One  nucleus  from  each  end,  the  two  being  called  "  polar 
nuclei,"  moves  toward  the  center  of  the  sac,  the  two  meet 
and  fuse  (Fig.  176,  at  right,  C),  and  a  single  large  nucleus 
is  the  result. 

The  three  nuclei  at  the  end  of  the  sac  nearest  the  micro- 
pyle  are  organized  into  cells,  each  being  definitely  sur- 
rounded by  cytoplasm,  but  there  is  no  wall  and  the  cells 
remain  naked  but  distinct.  These  three  cells  constitute 
the  egg-apparatus  (Fig.  176,  at  right,  A),  the  central  one, 
which  usually  hangs  lower  in  the  sac  than  the  others,  being 
the  egg,  the  two  others  being  the  synergids,  or  "helpers." 
Here,  therefore,  is  an  egg  without  an  archegonium,  a  dis- 
tinguishing feature  of  Angiosperms. 


SPERMATOPHYTES:  ANGIOSPERMS 


205 


The  three  nuclei  at  the  other  end  of  the  sac  are  also  or- 
ganized into  cells,  and  usually  have  walls.     These  cells  are 
known  as  antipodal  cells  (Fig.  176,  at  right, 
B).     The  large  nucleus  near  the  center  of  A 

L 


the  sac,  formed  by  the  fusion  of  the  two 


Fig.  176.  Lilium  Philadelphicxim,  showing  last  stages  of  germination  of  megaspore 
before  fertilization:  the  embryo  sac  to  the  left  contains  the  pair  of  nuclei  in  each 
end  in  a  state  of  division  preparatory  to  the  stage  represented  by  the  middle  figure, 
in  which  there  are  four  nuclei  at  each  end;  the  figure  to  the  right  shows  an  embryo- 
sac  containing  a  gametophyte  about  ready  for  fertilization,  with  the  egg  apparatus 
(A)  composed  of  the  two  synergids  and  egg  (central  and  lower),  the  three  antipo- 
dal cells  (B),  and  the  two  polar  nuclei  fusing  ( C)  to  form  the  primary  endosperm 
nucleus.— Caldwell. 

polar  nuclei,  is  known  as  the  primary  endosperm  nucleus 
or  the  definitive  nucleus. 
32 


206 


PLANT   STRUCTURES 


Fig.  177.  Fertilization  in  the  cotton  plant, 
a  Dicotyledon,  showing  the  pollen  tube  (P) 
passing  through  the  micropyle  and  con- 
taining a  single  sperm  (male  cell),  and  hav- 
ing entered  the  embryo-sac  is  in  contact 
with  one  of  the  synergids  (<S*)  on  its  way  to 
the  egg  (22).—  After  Duggar. 


Fertilization.  —  The 
pollen-tube,  carrying  the 
two  male  cells,  has  passed 
down  the  style  and  en- 
tered the  micropyle  (Fig. 
173).  It  then  reaches  the 
wall  of  the  embryo -sac, 
pierces  it,  and  is  in  con- 
tact with  the  egg-appa- 
ratus (Fig.  177).  When 
it  comes  near  the  egg,  the 
tip  of  the  tube  breaks  and 
the  two  male  cells  are  dis- 
charged into  the  embryo- 
sac.  One  male  cell  passes 
to  the  egg  and  the  two 
nuclei  fuse,  the  resulting 
cell  being  the  oospore, 
which  develops  the  em- 
bryo. The  other  male 
cell  passes  to  the  endo- 
sperm nucleus  and  fuses 
with  it,  the  cell  resulting 
from  this  triple  fusion  of 
a  male  cell  and  two  polar 
nuclei  developing  th  e 
endosperm  (Fig.  178). 
These  two  simultaneous 
acts  of  fertilization  are 
spoken  of  as  "  double  fer- 
tilization." 

Endosperm.  —  After 
fertilization,  the  primary 
endosperm  nucleus  begins 
a  series  of  divisions,  and 
as  a  result  the  sac  becomes 


SPERM ATOFIIYTES :    AN GIOS PERMS 


207 


more  or  less  filled  with  nutritive 
cells,  which  are  often  organized 
into  a  compact  tissue  (Fig.  179). 
These  nutritive  cells  do  not  cor- 
respond to  the  endosperm  of 
Gynmosperms,  although  they  re- 
receive  the  same  name.  In  Gym- 
nosperms the  endosperm  is  main- 
ly formed  before  fertilization  and 
is  the  nutritive  body  of  the  female 
gametophyte ;  while  in  Arigio- 
sperms  it  is  formed  after  fertiliza- 
tion and  is  probably  not  a  part 
of  the  gametophyte.  As  the 
endosperm  of  Angiosperms  is  a 
product  of  what  appears  to  be 
fertilization,  it  would  seem 
proper  to   regard    it   as    sporo- 

phyte  tissue,  but  its  real  character  is  still  under  discussion. 

The  antipodal  cells  probably  represent  nutritive  cells 

of  the  gametophyte.     Sometimes  they  disappear  very  soon 

after  they  are  formed;  but  sometimes  they  become  very 


Fig.  178.  End  of  embryo-sac  of 
Silphii/m,  showing  double  fer- 
tilization :  sy,  eynergid,  the 
other  having  been  destroyed  by 
the  pollen-tube ;  o,  egg  with 
coiled  male  cell  («/>,)  lying 
against  its  nucleus ;  e,  endo- 
sperm cell,  with  large  coiled 
male  cell  (sp9)  lying  against  it. 
—After  Land. 


Fig.  179.    One  end  of  the  embryo-sac  in  wake-robin  (Trillium),  showing  endosperm 
(shaded  cells)  in  which  a  young  embryo  is  imbedded. — After  Atkinson. 


208 


PLANT  STEUCTUKES 


active  and  even  divide  and  form  a  considerable  amount  of 
tissue,  which  usually  nourishes  the  embryo  until  endosperm 
tissue  is  developed,  and  then  becomes  disorganized;  or 
even  invades  the  tissue  of  the  nucellus. 

114.  Development  of  embryo. — While  the  endosperm  is 
forming,  the  oospore  has  germinated  and  the  sporophyte 
embryo  is  developing  (Fig.  180).  Usually  a  suspensor,  more 
or  less  distinct,  but  never  so  prominent  as  in  Gymnosperms, 

is  formed ;  at  the  end  of  it  the 
embryo  is  developed  (Fig.  181), 
which,  when  completed,  is  more 
or  less  surrounded  by  nourish- 
ing endosperm  (Fig.  183). 

The  two  groups  of  Angio- 
sperms  differ  widely  in  the  struc- 
ture of  the  embryo.  In  Mono- 
cotyledons the  axis  of  the  em- 
bryo develops  the  root-tip  at  one 
end  and  the  "  seed-leaf  "  (coty- 
ledon) at  the  other,  the  stem-tip 
arising  from  the  side  of  the  axis 
as  a  lateral  member  (Fig.  182). 
This  relation  of  organs  recalls 
the  embryo  of  Isoetes  (see  §  90). 
Naturally  there  can  be  but  one 
cotyledon  under  such  circum- 
stances, and  the  group  has  been 
named  Monocotyledons. 

In  Dicotyledons  the  axis  of 
the  embryo  develops  the  root-tip  at  one  end  and  the  stem- 
tip  at  the  other,  the  cotyledons  (usually  two)  appearing  as 
a  pair  of  opposite  lateral  members  on  either  side  of  the 
stem-tip  (Fig.  181).  This  recalls  the  relation  of  parts  in 
the  embryo  of  Selaginella  (see  §  89).  As  the  cotyledons 
are  lateral  members  their  number  may  vary.  In  Gymno- 
sperms, whose  embryos  are  of  this  type,  there  are  often 


Fig.  180.  Curved  embryo-sac  of 
arrowhead  (Sagitlaria),  show- 
ing in  the  upper  right  end  a 
young  embryo,  in  the  other 
end  the  antipodal  cells  cutoff 
by  a  partition,  and  scattered 
through  the  sac  a  few  free  en- 
dosperm cells. — After  Schafp- 
ner. 


SPERMATOPHYTES .    ANGIOSPERMS 


209 


several  cotyledons  in  a  cycle  (Fig.  154) ;  and  in  Dicotyle- 
dons there  may  be  one  or  several  cotyledons ;  but  as  a  pair 
of  opposite  cotyledons  is  almost  without  exception  in  the 
group,  it  is  named  Dicotyledons. 

The  axis  of  the  embryo  between  the  root-tip  and  the 
cotyledons  is  called  the  hypocotyl  (Figs.  154, 193, 194),  which 


Fig.  181.  Development  of  embryo  of  shepherd's  purse  (Capsella),  a  Dicotyledon: 
beginning  with  /,  tbs  youngest  stage,  and  following  the  sequence  to  VI,  the  old- 
est stage,  v  represents  the  suspensor,  c  the  cotyledons,  s  the  stem-tip,  w  the  root, 
h  the  root-cap.  Note  the  root-tip  at  one  end  of  the  axis  and  the  stem-tip  at  the 
other  between  the  cotyledons. — After  Hanstein. 

means  "  under  the  cotyledon,"  a  region  which  shows  pecul- 
iar activity  in  connection  with  the  escape  of  the  embryo 
from  the  seed.  Formerly  it  was  called  either  caulicle  or 
radicle.     In  Dicotyledons  the  stem-tip  between  the  coty- 


210 


PLANT   STKUCTURES 


ledons  often  organizes  the  rudiments  of  subsequent  leaves, 
forming  a  little  bud  which  is  called  the  plumule. 

Embryos  differ  much  as  to  com- 
pleteness of  their  development  within 
the  seed.  In  some  plants,  especially 
those  which  are  parasitic  or  sapro- 
phytic, the  embryo  is  merely  a  small 
mass  of  cells,  without  any  organiza- 
tion of  root,  stem,  or  leaf.  In  many 
cases  the  embryo  becomes  highly  de- 
veloped, the  endosperm  being  used 
up  and  the  cotyledons  stuffed  with 
food  material,  the  plumule  contain- 
ing several  well  -  organized  young 
leaves,  and  the  embryo  completely 
filling  the  seed  cavity.  The  com- 
mon bean  is  a  good  illustration  of 
this  last  case,  the  whole  seed  within 
the  integument  consisting  of  the  two 
large,  fleshy  cotyledons,  between 
which  lie  the  hypocotyl  and  a  plu- 
mule of  several  leaves. 

115.  The  seed.  —  As  in  Gymno- 
sperms,  while  the  processes  above 
described  are  taking  place  within 
the  ovule,  the  tissue  is  developing 
that  forms  the  hard  seed-coat  or  testa  (Fig.  183).  When 
this  hard  coat  is  fully  developed,  the  activities  within 
cease,  and  the  whole  structure  passes  into  that  condition  of 
suspended  animation  which  is  so  little  understood,  and 
which  may  continue  for  a  long  time. 

The  testa  is  variously  developed  in  seeds,  sometimes 
being  smooth  and  glistening,  sometimes  pitted,  sometimes 
rough  with  warts  or  ridges.  Sometimes  prominent  append- 
ages are  produced  which  assist  in  seed-dispersal,  as  the 
wings  in  Catalpa  or  Bignonia  (Fig.  184),  or  the  tufts  of 


Fig.  182.  Young  embryo  of 
water  plantain  (Alisma),  a 
Monocotyledon,  the  root 
being  organized  at  one 
end  (next  the  suspensor), 
the  single  cotyledon  (C) 
at  the  other,  and  the  stem- 
tip  arising  from  a  lateral 
notch  (v).  —  After  Han- 
stein. 


SPERMATOCYTES:   ANGIOSPERMS 


211 


Fig.  183.  The  two  figures  to  the  left  are  seeds  of  violet,  one  showing  the  black,  hard 
testa,  the  other  being  sectioned  and  showing  testa,  endosperm,  and  imbedded 
embryo;  the  figure  to  the  right  is  a  section  of  a  pepper  fruit  (Piper),  showing 
modified  ovary  wall  (pc),  seed  testa  (sc),  nucellus  tissue  (p),  endosperm  (en),  and 
embryo  (em).— After  Baillon. 

hair  on  the  seeds  of  milkweed,  cotton,  or  fireweed  (Fig. 
185).  For  a  fuller  account  of  the  methods  of  seed-dispersal 
see  Plant  Relations,  Chapter  VI. 


Fig.  184.    A  winged  seed  of  Bignonia.— After  Strasbitrger. 

116.  The  fruit.— The  effect  of  fertilization  is  felt  beyond 
the  boundaries  of  the  ovule,  which  forms  the  seed.  The 
ovary  is  also  involved,  and  becomes  more  or  less  modified. 
It  enlarges  more  or  less,  sometimes  becoming  remarkably 
enlarged.  It  also  changes  in  structure,  often  becoming 
hard  or  parchment-like.  In  case  it  contains  several  or 
numerous  seeds,  it  is  organized  to  open  in  some  way  and 
discharge  them,  as  in  the  ordinary  pods  and  capsules  (Fig. 
185).     In  case  there  is  but  one  seed,  the  modified  ovary 


212 


PLANT   STRUCTURES 


wall  may  invest  it  as  closely  as  another 
integument,  and  a  seed-like  fruit  is 
the  result — a  fruit  which  never  opens 
and  is  practically  a  seed.  Such  a 
fruit  is  known  as  an  akene,  and  is 
very  characteristic  of  the  greatest 
Angiosperm  family,  the  Composite, 
to  which  sunflowers,  asters,  golden- 
rods,  daisies,  thistles,  dandelions, 
etc.,  belong.  Dry  fruits  which  do 
not  open  to  discharge  the  seed  often 
bear  appendages  to  aid  in  dispersal 
by  wind  (Figs.  186, 187),  or  by  animals 
(Fig.  188). 

Capsules,  pods,  and  akenes  are  said 
to  be  dry  fruits,  but  in  many  cases 
fruits   ripen    fleshy.     In   the   peach, 
plum,  cherry,  and  all  ordinary  "  stone 
fruits,"  the  modified  ovary  wall  or- 
ganizes two  layers,  the  inner  being 
very  hard,  forming  the  "stone,"  the 
outer  being  pulpy  (Fig.  189),  or  vari- 
ously modified   (Fig.  190).      In  the  true  berries,  as  the 
grape,  currant,  tomato,  etc.,  the  whole  ovary  becomes  a 
thin-skinned  pulpy  mass  in  which  the  seeds  are  imbedded. 

In  some  cases 
the  effect  of  ferti- 
lization in  chang- 
ing structure  is 
felt  beyond  the 
ovary.  In  the  ap- 
ple, pear,  quince, 
and  such  fruits, 
the  pulpy  part  is 
the    modified 

Calyx  (one    Of  the         Fiq.  186.    .Winged  fruit  of  maple.— After  Keener. 


Fig.  185.  A  pod  of  firevveed 
{Epilobium)  opening  and 
exposing  its  plumed  seeds 
which  are  transported  by 
the  wind.— After  Beal. 


SPERMATOPHYTES:  ANGIOSPERMS 


213 


floral  leaves),  the  ovary  and  its  contained  seeds  being  repre- 
sented by  the  "core."  In  other  cases,  the  end  of  the  stem 
bearing  the  ovaries  (receptacle)  becomes  enlarged  and 
pulpy,  as  in  the  strawberry  (Fig.  191).  This  effect  some- 
times involves  even 
more  than  the 
parts  of  a  single 
flower,  a  whole 
flower-cluster, 
with  its  axis  and 
bracts,  becoming 
an  enlarged  pulpy 
mass,  as  in  the 
pineapple  (Fig. 
192). 

The        term 
"fruit,"  therefore, 


Fig.  187.  A  ripe  dandelion  head,  showing  the  mass  of 
plumes,  a  few  seed-like  fruits  (akenes)  with  their 
plumes  still  attached  to  the  receptacle,  and  two 
frllen  off.— After  Kerner. 


Fig.  188.  An  akene  of  beg- 
gar ticks,  showing  the  two 
barbed  appendages  which 
lay  hold  of  animals.— Af- 
ter Beal. 


Fig.  189.  To  the  left  a  section  of  a  peach  (fruit), 
showing  pulp  and  stone  formed  from  ovary  wall, 
and  the  contained  seed  (kernel) ;  to  the  right 
the  fruit  of  almond,  which  ripens  dry.— After 
Gray. 


PLANT  STRUCTURES 


•y  indefinite  one,  so  far  as  the  structures  it  includes 
concerned.     It  is  simply  an  effect  which  follows  fer- 
tilization, and  involves  more  or  less  of  the  structures  adja- 


Fig.  190.  Fruit  of  nutmeg  (Myristica) :  A,  section  of  fruit,  showing  seed  within  the 
heavy  wall ;  B,  section  of  seed,  showing  peculiar  convoluted  and  hard  endosperm 
(m)  in  which  an  embryo  (n)  is  imbedded.— After  Berg  and  Schmidt. 

cent  to  the  seeds.  As  has  been  seen,  this  effect  may  extend 
only  to  the  ovary  wall,  or  it  may  include  the  calyx,  or  it 
may  be  specially  directed  toward  the 
receptacle,  or  it  may  embrace  a  whole 
flower-cluster.  It  is  what  is  called  a 
physiological  effect  rather  than  a  defi- 
nite morphological  structure. 

117.  Germination  of  the  seed. — It 
has  been  pointed  out  (§  103)  that  the 
so-called  "germination  of  the  seed" 
is  not  true  germination  like  that  of 
spores.  It  is  the  awakening  and  es- 
cape of  the  young  sporophyte,  which 
has  long  before  passed  through  its 
germination  stage. 

By  various  devices  seeds  are  sepa- 
rated from  the  parent  plant,  are  dis- 
persed more  or  less  widely,  and  find 
lodgment.     If  the  lodgment  is  suitable,  there  are  many 
devices  for  burial,  such  as  twisting  stalks  and  awns,  bur- 


Fig.  191.  Fruit  of  straw- 
berry, showing  the  per- 
sistent calyx,  and  the  en- 
larged pulpy  receptacle 
in  which  numerous  sim- 
ple and  dry  fruits  (a- 
kenes)  are  imbedded. — 
After  Bailey. 


SPERMATOPHVTES:   ANGIOSPEKMS 


215 


rowing  animals,  etc.  The  period  of  rest  may  be  long  or 
short,  but  sooner  or  later,  under  the  influence  of  moisture, 
suitable  temperature,  and  oxygen  the  quiescent  seed  begins 
to  show  signs  of  life. 

The  sporophyte  within  begins  to  grow,  and  the  seed 
coat  is  broken  or  penetrated  through  some  thin  spot  or 

\s   *    A/,// 


Pig.  192.  Pineapple:  A,  the  cluster  of  fruits  forming  the  so-called  "fruit";  B,  single 
flower,  showing  small  calyx  and  more  prominent  corolla;  C,  section  of  flower, 
showing  the  floral  organs  arising  above  the  ovary  (epigynous).—  A,  B  after  Koch; 
C  after  Lb  Maout  and  Decaisnb. 

opening.  The  root-tip  emerges  first,  is  protruded  still 
farther  by  the  rapid  elongation  of  the  hypocotyl,  soon 
curves  toward  the  earth,  penetrates  the  soil,  and  sending 
out  rootlets,  becomes  anchored.     After  anchorage  in  the 


216 


PLANT    STRUCTURES 


soil,  the  hypocotyl  again  rapidly  elongates  and  develops  a 
strong  arch,  one  of  whose  limbs  is  anchored,  and  the  other 
is  pulling  upon  the  cotyledons  (Fig.  193).  This  pull  finally 
frees  the  cotyledons,  the  hypocotyl  straightens,  the  cotyle- 


Fig.  193.  Germination  of  the  garden  bean,  showing  the  arch  of  the  hypocotyl  above 
ground,  its  pull  on  the  seed  to  extricate  the  cotyledons  and  plumule,  and  the  final 
straightening  of  the  stem  and  expansion  of  the  young  leaves.— After  Atkinson. 

dons  are  spread  out  to  the  air  and  light,  and  the  young 
sporophyte  has  become  independent  (Fig.  194). 

In  the  grain  of  corn  and  other  cereals,  so  often  used  in 
the  laboratory  as  typical  Monocotyledons,  but  really  excep- 
tional ones,  the  embryo  escapes  easily,  as  it  is  placed  on 
one  side  of  the  seed  near  the  surface.  The  hypocotyl  and 
stem  split  the  thin  covering,  and  the  much-modified  cotyle- 
don is  left  within  the  grain  to  absorb  nourishment. 

In  some  cases  the  cotyledons  do  not  escape  from  the 
seed,  either  being  distorted  with  food  storage  (oak,  buck- 
eye, etc.),  or  being  retained  to  absorb  nourishment  from 
the  endosperm  (palms,  grasses,  etc.).  In  such  cases  the 
stem-tip  is  liberated  by  the  elongation  of  the  petioles  of  the 


SPERMATOPHYTES :   ANGIOSPERMS 


217 


cotyledons,  and  the  seed  coat  containing  the  cotyledons 
remains  like  a  lateral  appendage  upon  the  straightened  axis. 

It  is  also  to  be  observed  in 
many  cases  that  the  young  root 
system,  after  gripping  the  soil, 
contracts,  drawing  the  young 
plant  deeper  into  the  ground. 

118.  Summary  from  Angio- 
sperms. — At  the  beginning  of  this 
chapter  (§  107)  the  characters  of 
the  Gymnosj)erms  were  summar- 
ized which  distinguished  them 
from  Angiosperms,  whose  con- 
trasting characters  may  be  stated 
as  follows  : 

(1)  The  microspore  (pollen- 
grain),  chiefly  by  insect  pollina- 
tion, is  brought  into  contact  with 
the  stigma,  which  is  a  receptive 

.  region  on  the  surface  of  the  car- 
pel, and  there  develops  the  pollen- 
tube,  which  penetrates  the  style 
to  reach  the  ovary  cavity  which 
contains  the  ovules  (megasporan- 
gia).  The  impossibility  of  con- 
tact between  pollen  and  ovule  im- 
plies inclosed  ovules  and  hence 
seeds,  and  therefore  the  name 
"  Angiosperm." 

(2)  The  female  gametophyte 
at  the  time  of  fertilization  con- 
sists of  only  a  few  free  nuclei  and 
cells,  usually  seven  in  number. 

(3)  The  female  gametophyte  produces  no  archegonia, 
but  a  single  naked  egg. 


Fig.  194.  Seedling  of  hornbeam 
(Oarpintts),  Bhowing  primary 
root  (hiv)  bearing  rootlets  (ato) 
upon  which  are  numerous 
root  hairs  (r),  hypocotyl  (A), 
cotyledons  (c),  young  stem 
(<?),  and  first  (I)  and  second 
(/')  true  leaves.— After  Sciiim- 

PER. 


CHAPTER    XIII 

THE    FLOWER 

119.  General  characters. — In  general  the  flower  may  be 
regarded  as  a  modified  branch  of  the  sporophyte  stem  bear- 
ing sporophylls  and  usually  floral  leaves.  Its  representa- 
tive among  the  Pteridophytes  and  Gymnosperms  is  the  stro- 
bilus,  which  has  sporophylls  but  not  floral  leaves.  Among 
Angiosperms  it  begins  in  a  simple  and  somewhat  indefinite 
way,  gradually  becomes  more  complex  and  modified,  until 
it  appears  as  an  elaborate  structure  very  efficient  for  its 
purpose. 

This  evolution  of  the  flower  has  proceeded  along  many 
lines,  and  has  resulted  in  endless  diversity  of  structure. 
These  diversities  are  largely  used  in  the  classification  of 
Angiosperms,  as  it  is  supposed  that  near  relatives  are  indi- 
cated by  similar  floral  structures,  as  well  as  by  other  fea- 
tures. The  significance  of  these  diversities  is  supposed  to 
be  connected  with  securing  proper  pollination,  chiefly  by 
insects,  and  favorable  seed  distribution. 

Although  the  evolution  of  flowers  has  proceeded  along 
several  lines  simultaneously,  now  one  feature  and  now 
another  being  emphasized,  it  will  be  clearer  to  trace  some 
of  the  important  lines  separately. 

120.  Floral  leaves. — In  the  simplest  flowers  floral  leaves 
do  not  appear,  and  the  flower  is  represented  only  by  the 
sporophylls.  Both  kinds  of  sporophylls  may  be  associated, 
in  which  case  the  flower  is  said  to  be  'perfect  (Fig.  195) ;  or 
they  may  not  both  occur  in  the  same  flower,  in  which  case 
one  flower  is  staminate  and  the  other  pistillate  (Fig.  196). 

218 


THE  FLOWER 


219 


When  the  floral  leaves  first  appear  in  connection  with 
the  sporophylls  they  are  inconspicuous,  scale-like  bodies. 
In  higher  forms  they  become  more  prominent  and  inclose 


Fig.  195.   Lizard's  tail  (Saururus):  A,  tip  of  branch 
bearing  leaves  and  elongated  cluster  of  flowers; 

B,  a  single  naked  flower  from  A,  showing  sta- 
mens and  four  spreading  and  stigmatic  styles; 

C,  flower  from  another  species,  showing  sub- 
tending bract,  absence  of  floral  leaves,  seven 
stamens,  and  a  syncarpous  pistil ;  the  flowers 
naked  and  perfect.— After  Engler. 


Fig.  196.  Naked  flowers  of  dif- 
ferent willows  ISalix),  each 
from  the  axil  of  a  bract : 
a,  b,  c,  staminate  flowers ; 
d,  e,  /,  pistillate  flowers,  the 
pistil  composed  of  two  car- 
pels (syncarpous).  —  After 
Warming. 


Fig.  197.  Flower  of  calamus 
(Acorus),  showing  simple 
perianth,  stamens,  and  syn- 
carpous pistil :  a  hypogynous 
flower  without  differentiation 
of  calyx  and  corolla.— After 
Engler. 


Fig.  199.  Common  flax  (Limtm) : 
A.  entire  flower,  showing  calyx 
and  corolla  ;  I?,  floral  leaves  re- 
moved, showing  stamens  and 
syncarpous  pistil ;  C,  a  mature 
Fig.  198.    Flowers  of  elm  (I'lmt/s) :  A,  branch  capsule  splitting   open. —After 

bearing  clusters  of  flow-ers  and  scaly  buds  ;  Schimper. 

B,  single  flower,  showing  simple  perianth 
and  stamens,  being  a  staminate  flower;   V, 

flower  showing  perianth,  stamens,  and  the  two  divergent  styles  stigmatic  on  inner 
surface,  being  a  perfect  flower;  D,  section  through  perfect  flower,  showing  peri- 
anth, stamens,  and  pistil  with  two  loculi  each  with  a  single  ovule. — After  Englek. 


Fig.  200.  A  flower  of  peony,  showing  the  four  sets  of  floral  organs:  k,  the  sepals,  to- 
gether called  the  calyx;  c,  the  petals,  together  called  the  corolla;  a,  the  numerous 
stamens;  g,  the  two  carpels,  which  contain  the  ovules.— After  Strasburger. 


THE  FLOWER 


221 


the  young  sporophylls,  but  they  are  all  alike,  forming  what 
is  called  the  perianth  (Figs.  197,  198). 

In  still  higher  forms  the  perianth  differentiates,  the 
inner  floral  leaves  become  more  delicate  in  texture,  larger 
and  generally  brightly  colored  (Fig.  199,  ^1).  The  outer 
set  may  remain  scale-like,  or  become  like  small  foliage 
leaves.  When  the  dif- 
ferentiation of  the  peri- 
anth is  distinct,  the 
outer  set  of  floral  leaves 
is  called  the  calyx,  each 
leaf  being  a  sepal ;  the 
inner  set  is  the  corolla, 
each  leaf  being  a  petal 
(Fig.  200).  Sometimes, 
as  in  the  lily,  all  the 
floral  leaves  become 
uniformly  large  and 
brightly  colored,  in 
which  case  the  term 
perianth  is  retained 
(Fig.  201).  In  other 
cases,  the  calyx  may  be 
the  large  and  colored 
set,  but  whenever  there 
is  a  clear  distinction 
between  sets,  the  outer 
is  the  calyx,  the  inner 
the  corolla. 

Both  floral  sets  may 
not  appear,  and  it  has 
become  the  custom  to 
regard  the  missing  set 
as  the  corolla,  such 
flowers  being  called 
apetalous,  meaning 
33 


Fig.  201. —An  easter-lily.  a  Monocotyledon, 
showing  perianth  (a),  stamens  (b),  stigma  (c), 
flower  bud  (d),  and  a  carpel  after  the  peri- 
anth has  fallen  (I),  with  its  knob-like  stigma, 
long  style,  and  slender  ovary.— Caldwell. 


222  PLANT  STRUCTURES 

"  without  petals."  It  is  not  always  possible  to  tell  whether 
a  flower  is  apetalous — that  is,  whether  it  has  lost  a  floral 
set  which  it  once  had — or  is  simply  one  whose  perianth  has 
not  yet  differentiated,  in  which  case  it  would  be  a  "primi- 
tive type." 

The  line  of  evolution,  therefore,  extends  from  flowers 
without  floral  leaves,  or  naked  flowers,  to  those  with  a  dis- 
tinctly differentiated  calyx  and  corolla. 

121.  Spiral  to  cyclic  flowers. — In  the  simplest  flowers  the 
sporophylls  and  floral  leaves  (if  any)  are  distributed  about 
an  elongated  axis  in  a  spiral,  like  a  succession  of  leaves. 
That  part  of  the  axis  which  bears  the  floral  organs  is  for 
convenience  called  the  receptacle  (Fig.  202).     As  the  recep- 


.» 

Fig.  202.  A  buttercup  (Ranunculus):  a,  complete  flower,  showing  sepals,  petals,  sta- 
mens, and  head  of  numerous  carpels  on  a  large  receptacle;  b,  section  showing 
relation  of  parts;  a  hypogynous,  polypetalous,  apocarpous,  actinomorphic  flower. 
—After  Baillon. 

tacle  is  elongated  and  capable  of  continued  growth,  an  in- 
definite number  of  each  floral  organ  may  appear,  especially 
of  the  sporophylls.  With  the  spiral  arrangement,  there- 
fore, there  is  no  definiteness  in  the  number  of  floral  organs  ; 
there  may  be  one  or  very  many  floral  leaves,  or  stamens,  or 
carpels.  The  spiral  arrangement  and  indefinite  numbers 
are  features  of  the  ordinary  strobilus,  and  therefore  such 
flowers  are  regarded  as  more  primitive  than  the  others. 

In  higher  forms   the   receptacle  becomes  shorter,  the 
spiral  more  closely  coiled,  until  finally  the  sets  of  organs 


THE   FLOWER  223 

appear  to  be  thrown  into  rosettes  or  cycles.  This  change 
does  not  necessarily  affect  all  the  parts  simultaneously. 
For  example,  in  the  common  buttercup  the  sepals  and 
petals  are  nearly  in  cycles,  while  the  carpels  are  spirally 
arranged  and  indefinitely  numerous  on  the  head-like  recep- 
tacle (Fig.  202).     On  the  other  hand,  in  the  common  water- 


Fig.  203.     Flower  of  water-lily  (Nymphcea),  showing  numerous  petals  and  stamens. — 
After  Caspart. 

lily  the  petals  and  stamens  are  spiral,  and  indefinitely  re- 
peated, while  the  sepals  and  carpels  are  approximately 
cyclic  (Fig.  203). 

Finally,  in  the  highest  forms,  all  the  floral  organs  are 
in  definite  cycles,  and  there  is  no  indefinite  repetition  of 
any  part.  All  through  this  evolution  from  the  spiral  to  the 
cyclic  arrangement  there  is  constantly  appearing  a  tend- 
ency to  "settle  down  "to  certain  definite  numbers.  When 
the  complete  cyclic  arrangement  is  finally  established  these 
numbers  are  established,  and  they  are  characteristic  of 
great  groups.  In  cyclic  Monocotyledons  there  are  nearly 
always  just  three  organs  in  each  cycle,  forming  what  is 
called  a  trimerous  flower  (Fig.  204) ;  while  in  cyclic  Dicot- 


224 


PLANT   STKUCTUKES 


yledons  the  number  five  prevails,  hut  often  four  appears, 
forming  pentamerous  or  tetramerous  flowers  (Fig.  199). 
This  does  not  mean  that  there  are  necessarily  just  three, 
four,  or  five  of  each  organ  in  the  flower,  for  there  may  be 
two  or  more  cycles  of  some  one  organ.  For  example,  in  the 
common  lily  there  are  six  floral  leaves  in  two  sets,  six  sta- 
mens in  two  sets,  and  three  carpels  (Fig.  204). 

In  the  cyclic  flowers  it  is  also  to  be  noted  that  each  set 
alternates  with  the  next  set  outside  (Fig.  204).     The  petals 

are  not  directly  opposite  the  se- 
pals, but  are  opposite  the  spaces 
between   sepals ;   the  stamens  in 
^J^<w>'>^  \  turn  alternate  with  the  petals;  if 

f  c9/^\u>  i\  there  is  a  second  set  of  stamens, 

\  /v,  V&})  „  '  1  it  alternates  with  the  outer  set, 

and  so  on.  If  two  adjacent  sets 
are  found  opposing  one  another, 
it  is  usually  due  to  the  fact  that 
a  set  between  has  disappeared. 
For  example,  if  a  set  of  stamens 
is  opposite  the  set  of  petals,  either 
an  outer  stamen  set  or  an  inner 
petal  set  has  disappeared. 

This  line  of  evolution,  there- 
fore, extends  from  flowers  whose 
parts  are  spirally  arranged  upon 
an  elongated  receptacle  and  in- 
definite in  number,  to  those  whose  parts  are  in  cycles  and 
definite  in  number. 

122.  Hypogynous  to  epigynous  flowers. — In  the  simpler 
flowers  the  sepals,  petals,  and  stamens  arise  from  beneath 
the  ovary  (Figs.  197,  202,  205,  1).  As  in  such  cases  the 
ovary  or  ovaries  may  be  seen  distinctly  above  the  origin 
(insertion)  of  the  other  parts,  such  a  flower  is  often  said  to 
have  a  "superior  ovary."  The  more  usual  term,  however, 
is  hypogynous,  meaning  in  effect  "  under  the  ovary,"  refer- 


Fig.  304.  Diagram  of  such  a 
flower  as  the  lily,  showing  re- 
lation of  parts  :  uppermost 
organ  is  the  bract  in  the  axil 
of  which  the  flower  occurs  ; 
black  dot  below  indicates  po- 
sition of  stem  ;  floral  parts  in 
threes  and  in  five  alternating 
cycles  (two stamen  Bets),  being 
a  trimerous.  pentacyclic  flow- 
er.— After  Schimper. 


THE    FLOWER 


225 


ring  to  the  fact  that  the  insertion  of  the  other  parts  is 
under  the  ovary. 

Hypogyny  is  very  largely  displayed  among  flowers,  but 
there  is  to  be  observed  a  tendency  in  some  to  carry  the 
insertion  of  the  outer  parts  higher  up.  When  the  outer 
parts  arise  from  the  rim  of  an  urn-like  outgrowth  from  the 


Fig.  905.  Flowers  of  Rose  family:  1,  a  hypogynous 
flower  of  PotentUla,  sepals,  petals,  and  stamens 
arising  from  beneath  the  head  of  carpels;  2,  a 
perigynous  flower  of  Alchemilla,  sepals,  petals, 
and  stamens  arising  from  rim  of  urn-like  pro- 
longation of  the  receptacle,  which  surrounds  the 
carpel ;  3,  an  epigynous  flower  of  the  common 
apple,  in  which  all  the  parts  seem  to  arise  from 
the  top  of  the  ovary,  two  of  whose  loculi  are 
seen.— After  Focke. 


receptacle,  which  surrounds  the  pistil  or  pistils,  the  flower 
is  said  to  be  perigynous  (Figs.  205,  #,  206),  meaning  "  around 
the  pistil."  Finally,  the  insertion  is  carried  above  the  ovary, 
and  sepals,  petals,  and  stamens  seem  to  arise  from  the  top 
of  the  ovary  (Fig.  205,  3),  such  a  flower  being  epigynous, 
the  outer  parts  appearing  "'upon  the  ovary."  In  such  a 
case  the  ovary  does  not  appear  within  the  flower,  but  below 
it  (Figs.  205,  252,  261),  and  the  flower  is  often  said  to  have 
an  "inferior  ovary." 

123.  Apocarpous  to  syncarpous  flowers. — In   the   simpler 
flowers  the  carpels  are  entirely  distinct,  each  carpel  organ- 


226 


PLANT   STRUCTURES 


izing  a  simple  pistil,  a  single  flower  containing  as  many 
pistils  as  there  are  carpels,  as  in  the  buttercups  (Figs. 
200,  202).  Such  a  flower  is  said  to  be  apocarpous,  meaning 
"carpels   separate."     There  is  a   very   strong  tendency, 


Fig.  206.  Sweet-scented  shrub  (Calycanthus):  A,  tip  of  branch  bearing  flowers;  B, 
section  through  flower,  showing  numerous  floral  leaves,  stamens,  and  carpels,  and 
also  the  development  of  the  receptacle  about  the  carpels,  making  a  perigynous 
flower.— After  Thiebatjlt. 

however,  for  the  carpels  of  a  flower  to  organize  together 
and  form  a  single  compound  pistil.  In  such  a  flower  there 
may  be  several  carpels,  but  they  all  appear  as  one  organ 
(Figs.  195,  (7,  197,  198,  D,  199,  B),  and  the  flower  is  said 
to  be  syncarpous,  meaning  "carpels  together." 

124.  Polypetalous  to  sympetalous  flowers. — The  tendency 
for  parts  of  the  same  set  to  coalesce  is  not  confined  to  the 
carpels.  Sepals  often  coalesce  (Fig.  208),  and  sometimes 
stamens,  but  the  coalescence  of  petals  seems  to  be  more 
important.  Among  the  lower  forms  the  petals  are  entirely 
separated  (Figs.  199,  A,  202,  203,  207),  a  condition  which 


THE  FLOWER 


227 


has  received  a  variety  of  names,  but 
probably  the  most  common  is  poly- 
pet  alous,  meaning  "petals  many," 
although  eleutheropetalotis,  meaning 
"petals  free,"  is  much  more  to  the 
point. 

In  the  highest  Angiosperms,  how- 
ever, the  petals  are  coalesced,  form- 
ing a  more  or  less  tubular  organ 
(Figs.  208-210).  Such  flowers  are 
said  to  be  sympetalous,  meaning 
"petals  united."  The  words  gamo- 
petalous  and  monopetalous  are  also 

much  used,  but  all  three  words  refer  to  the  same  condition 
of  the  flower.     Often  the  sympetalous  corolla  is  differenti- 


Fio.  207.  Flower  of  straw- 
berry, showing  sepals,  pet- 
als, numerous  stamens, 
and  head  of  carpels  ;  the 
flower  is  actinomorphic, 
hypogynous,  and  with  no 
coalescence  of  parts. — Af- 
ter Bailey. 


Fig.  208.  A  flower  of  the  tobacco  plant:  a,  a  complete  flower,  showing  the  calyx  with 
its  sepals  blended  below,  the  ftinnelform  corolla  made  up  of  united  petals,  and  the 
stamens  just  showing  at  the  mouth  of  the  corolla  tube;  b,  a  corolla  tube  split  open 
and  showing  the  five  stamens  attached  to  it  near  the  base;  c,  a  syncarpous  pistil 
made  up  of  two  carpels,  showing  ovary,  style,  and  stigma.— After  Strasburger. 


228 


PLANT   STRUCTUKES 


ated  into  two  regions  (Fig.  210,  b),  a  more  or  less  tubular 
portion,  the  tube,  and  a  more  or  less  flaring  portion,  the  limb. 

125.  Actinomorphic 
to  zygomorphic  flow- 
ers.— In  the  simpler 
flowers  all  the  mem- 
bers of  any  one  cycle 
are  alike  ;  the  petals 
are  all  alike,  the 
stamens  are  all  alike, 
etc.  Looking  at  the 
center  of  the  flower, 
all  the  parts  are  re- 
peated about  it  like 
the  parts  of  a  radi- 
ate animal.  Such  a 
flower  is  actinomor- 
phic, meaning  "ra- 
diate," and  is  often 
called  a  "  regular 
flower."  Although 
the  term  actinomor- 
phic •strictly  applies  to  all  the  floral  organs,  it  is  especially 
noteworthy  in  connection  with  the  corolla,  whose  changes 
will  be  noted. 


Pig.  209.  Flower  of  morning-glory  (Ipomcea),  with 
sympetalous  corolla  split  open,  showing  the  five 
attached  stamens,  and  the  superior  ovary  with 
prominent  style  and  stigma  ;  the  flower  is  hy- 
pogynous,  sympetalous,  and  actinomorphic. — 
After  Mkissner. 


Fig.  210.  A  group  of  sympetalous  flower  forms:  a,  a  flower  of  harebell,  showing  a 
bell-shaped  corolla;  b,  a  flower  of  phlox,  showing  a  tube  and  spreading  limb;  c,  a 
flower  of  dead-nettle,  showing  a  zygomorphic  two-lipped  corolla;  d,  a  flower  of 
toad-flax,  showing  a  two-lipped  corolla,  and  also  a  spur  formed  by  the  base  of  the 
corolla;  e,  a  flower  of  the  snapdragon,  showing  the  two  lips  of  the  corolla  closed. 
—After  Gray. 


THE  FLOWER 


229 


In  many  cases  the  petals  are  not  all  alike,  and  the  radi 
ate  character,  with  its  similar  parts  repeated  about  a  cen 
ter,  is  lost.  In  the 
common  violet,  for 
example,  one  of  the 
petals  develops  a  spur 
(Fig.  211)  ;  in  the 
sweet  pea  the  petals 
are  remarkably  un- 
like, one  being  broad 
and  erect,  two  small- 
er and  drooping 
downward,  and  the 
other  two  much  modi- 
fied to  form  together 
a  boat-like  structure 
which  incloses  the 
sporophylls.  Such  flowers  are  called  zygomorphic,  meaning 
"  yoke-form,"  and  they  are  often  called  "  irregular  flowers." 

When   zygomorphic   flowers  are  also  sympetalous  the 
corolla  is  often  curiously  shaped.     A  very  common  form 


Fig.  811.  The  pansy  (Viola  tricolor) :  A,  section 
showing  sepals  (/,  I'),  petals  (c)  one  of  which 
produces  a  spur  (cs),  the  flower  being  zygomor- 
phic; B,  mature  fruit  (a  capsule)  and  persistent 
calyx  (k);  C,  the  three  boat-shaped  valves  of 
the  fruit  open,  most  of  the  seeds  (s)  having 
been  discharged.— After  Sachs. 


Fig.  212.  Flower  of  a  mint  (Mentha  aquatica):  A,  the  entire  flower,  showing  calyx 
of  united  sepals,  unequal  petals,  stamens,  and  style  with  two  stigma  lobes;  B,  a 
corolla  split  open,  showing  petals  united  and  the  four  stamens  attached  to  the 
tube;  the  flower  is  sympetalous  and  zygomorphic— After  Werming. 


230 


PLANT  STRDCTUKES 


Fig.  213.  Flower  of  a  Labiate  (Teucrium), 
showing  the  calyx  of  coalesced  sepals, 
the  sympetalous  and  two-lipped  (bilabi- 
ate) corolla  with  three  petals  (middle  one 
largest)  in  the  lower  lip  and  two  small 
ones  in  the  upper,  and  the  stamens  and 
style  emerging  through  a  slit  on  the  up- 
per side  of  the  tube;  a  sympetalous  and 
zygomorphic  flower.— After  Briquet. 


is  the  bilabiate,  or  "  two-lipped,"  in  which  two  of  the  petals 
usually  organize  to  form  one  lip,  and  the  other  three  form 

the  other  lip  (Figs.  210, 
c,  d,  e,  212,  213).  The  two 
lips  may  be  nearly  equal, 
the  upper  may  stand  high 
or  overarch  the  lower,  the 
lower  may  project  more  or 
less  conspicuously,  etc. 

126.  Inflorescence.— 
Very  often  flowers  are  soli- 
tary, either  on  the  end  of 
a  stem  or  branch  (Figs. 
231,  236),  or  in  the  axil 
of  a  leaf  (Fig.  258).  But 
such  cases  grade  insensibly  into  others  where  a  definite 
region  of  the  plant  is  set  aside  to  produce  flowers  (Figs. 
253,  260).  Such  a  region  forms  what  is  called  the  inflo- 
rescence. The  various  ways  in  which  flowers  are  arranged 
in  an  inflorescence  have  received  technical  names,  but  they 
do  not  enter  into  our  purpose  here.  They  are  simply  dif- 
ferent ways  in  which  plants  seek  to  display  their  flowers 
so  as  to  favor  pollination  and  seed  distribution. 

There  are  several  tendencies,  however,  which  may  be 
noted.  Some  groups  incline  to  loose  clusters,  either  elon- 
gated (Fig.  260)  or  flat-topped  (Fig.  253)  ;  others  prefer 
large  and  often  solitary  flowers  (Fig.  258)  to  a  cluster  of 
smaller  ones  ;  but  in  the  highest  groups  there  is  a  distinct 
tendency  to  reduce  the  size  of  the  flowers,  increase  their 
number,  and  mass  them  into  a  compact  cluster.  This  ten- 
dency reaches  its  highest  expression  in  the  greatest  family 
of  the  Angiosperms,  the  Composite,  of  which  the  sunflower 
or  dandelion  can  be  taken  as  an  illustration  (Figs.  261, 262), 
in  which  numerous  small  flowers  are  closely  packed  together 
in  a  compact  cluster  which  resembles  a  single  large  flower. 
It  does  not  follow  that  all  very  compact  inflorescences  in- 


THE  FLOWER  231 

dicate  plants  of  high  rank,  for  the  cat-tail  flag  (Fig.  221) 
and  many  grasses  have  very  compact  inflorescences,  and 
they  are  supposed  to  be  plants  of  low  rank.  It  is  to  be 
noted,  however,  that  the  very  highest  groups  have  settled 
upon  this  as  the  best  type  of  inflorescence. 

127.  Summary. — In  tracing  the  evolution  of  flowers, 
therefore,  the  following  tendencies  become  evident :  (1) 
from  naked  flowers  to  those  with  distinct  calyx  and  corolla  ; 
(2)  from  spiral  arrangement  and  indefinite  numbers  to  cyclic 
arrangement  and  definite  numbers ;  (3)  from  hypogynous 
to  epigynous  flowers ;  (4)  from  apocarpous  to  syncarpous 
pistils  ;  (5)  from  polypetalous  to  sympetalous  corollas  ;  (6) 
from  actinomorphic  or  regular  to  zygomorphic  or  irregular 
flowers  ;  (7)  from  loose  to  compact  inflorescences. 

These  various  lines  appear  in  all  stages  of  advancement 
in  different  flowers,  so  that  it  would  be  impossible  to  deter- 
mine the  relative  rank  in  all  cases.  However,  if  a  flower 
is  naked,  spiral,  with  indefinite  numbers,  hypogynous,  and 
apocarpous,  it  would  certainly  rank  very  low.  On  the  con- 
trary, the  flowers  of  the  Composite  have  calyx  and  corolla, 
are  cyclic,  epigynous,  syncarpous,  sympetalous,  often  zygo- 
morphic, and  are  in  a  remarkably  compact  inflorescence, 
indicating  the  highest  possible  combination  of  characters. 

128.  Flowers  and  insects.  —  The  adaptations  between 
flowers  and  insects,  by  which  the  former  secure  pollination 
and  the  latter  food,  are  endless.  Many  Angiosperm  flowers, 
especially  those  of  the  lower  groups,  are  still  anemophilous, 
as  are  the  Gymnosperms,  but  most  of  them,  by  the  presence 
of  color,  odor,  and  nectar,  indicate  an  adaptation  to  the 
visits  of  insects.  This  wonderful  chapter  in  the  history  of 
plants  will  be  found  discussed,  with  illustrations,  in  Plant 
Relations,  Chapter  VII. 


CHAPTER  XIV 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


129.  Contrasting  characters. — The  two  great  groups  of 
Angiosperms  are  quite  distinct,  and  there  is  usually  no  dif- 
ficulty in  recognizing  them.  The  monocotyledons  are 
usually  regarded  as  the  older  and  the  simpler  forms,  and 
are  represented  by  about  twenty  thousand  species.  The 
Dicotyledons  are  much  more  abundant  and  diversified,  con- 
taining about  eighty  thousand  species,  and  form  the  domi- 
nant vegetation  almost  everywhere. 
The  chief  contrasting  characters 
may  be  stated  as  follows  : 

Monocotyledons.  —  (1)  Embryo 
with  terminal  cotyledon  and  lat- 
eral stem-tip.  This  character  is 
practically  without  exception. 

(2)  Vascular  bundles  of  stem 
scattered  (Fig.  214).  This  means 
that  there  is  no  annual  increase  in 
the  diameter  of  the  woody  stems, 
and  no  extensive  branching,  but 
to  this  there  are  some  exceptions. 

(3)  Leaf  veins  forming  a  closed 
system  (Fig.  215,  figure  to  left). 
As  a  rule  there  is  an  evident  set 

of  veins  which  run  approximately  parallel,  and  intricately 
branching  between  them  is  a  system  of  minute  veinlets  not 
readily  seen.     The  vein  system  does  not  end  freely  in  the 
232 


Fig.  214.  Section  of  stem  of 
corn,  showing  the  scattered 
bundles,  indicated  by  black 
dots  in  cross-section,  and  by 
lines  in  longitudinal  section. 
—From  "Plant  Relations." 


MONOCOTYLEDONS   AND   DICOTYLEDONS  233 

margin  of  the  leaf,  but  forms  a  "  closed  venation/'  so  that 
the  leaves  usually  have  an  even  (entire)  margin.     There 

are  some  notable  exceptions 
to  this  character. 

(4)   Cyclic  flowers  trim- 
erous.    The  "  three-parted  " 


Pig.  215.  Two  types  of  leaf  venation:  the  figure  to  the  left  is  from  Solomon's  seal, 
a  Monocotyledon,  and  shows  the  principal  veins  parallel,  the  very  minute  cross 
veinlets  heing  invisible  to  the  naked  eye;  that  to  the  right  is  from  a  willow,  a 
Dicotyledon,  and  shows  netted  veins,  the  main  central  vein  (midrib)  sending  out 
a  series  of  parallel  branches,  which  are  connected  with  one  another  by  a  network 
of  veinlets. — After  Ettingshausen. 


flowers  of  cyclic  Monocotyledons  are  quite  characteristic, 
but  there  are  some  trimerous  Dicotyledons. 

Dicotyledons. — (1)  Embryo  with  lateral  cotyledons  and 
terminal  stem-tip. 

(2)  Vascular  bundles  of  stem  forming  a  hollow  cylinder 
(Fig.  216,  w).     This  means  an  annual  increase  in  the  diam- 


234 


PLANT  STRUCTUKES 


Fig.  216.  Section  across  a  young  twig  of 
box  elder,  showing  the  four  stem  regions: 
e,  epidermis,  represented  by  the  heavy 
bounding  line;  c,  cortex;  w,  vascular  cyl- 
inder; p,  pith.— From  "Plant  Relations." 


eter  of  woody  stems  (Fig. 
217,  w),  and  a  possible 
increase  of  the  branch 
system  and  foliage  dis- 
play each  year. 

(3)  Leaf  veins  form- 
ing an  open  system  (Fig. 
215,  figure  to  right). 
The  network  of  smaller 
veinlets  between  the 
larger  veins  is  usually 
very  evident,  especially 
on  the  under  surface  of 
the  leaf,  suggesting  the 
name  "net- veined" 
leaves,  in  contrast  to  the  "  parallel -veined  "  leaves  of  Mono- 
cotyledons. The  vein  system  ends  freely  in  the  margin  of 
the  leaf,  forming  an  "  open  venation."  In  consequence  of 
this,  although  the  leaf 
may  remain  entire,  it 
very  commonly  be- 
comes toothed,  lobed, 
and  divided  in  various 
ways.  Two  main  types 
of  venation  may  be 
noted,  which  influence 
the  form  of  leaves.  In 
one  case  a  single  very 
prominent  vein  {rib) 
runs  through  the  mid- 
dle of  the  blade,  and 
is  called  the  midrib. 
From  this  all  the  mi- 
nor veins  arise  as 
branches  (Figs.  218, 
219),  and  such  a  leaf 


Fig.  217.  Section  across  a  twig  of  box  elder 
three  years  old,  showing  three  annual  rings, 
or  growth  rings,  in  the  vascular  cylinder;  the 
radiating  lines  (m)  which  cross  the  vascular 
region  (w)  represent  the  pith  rays,  the  princi- 
pal ones  extending  from  the  pith  to  the  cor- 
tex (c).— From  "  Plant  Relations." 


MONOCOTYLEDONS   AND    DICOTYLEDONS 


235 


is  said  to  be  pinnate  or  pinnately  veined,  and  inclines  to 
elongated  forms.  In  the  other  case  several  ribs  of  eqnal 
prominence  enter  the  blade  and  diverge  through  it  (Fig. 
218).  Such  a  leaf  is  palmate  or  palmately  veined,  and  in- 
clines to  broad  forms. 

(4)   Cyclic   flowers  pentamerous  or  tetramerous.     The 
flowers  "  in  fives  "  are  greatly  in  the  majority,  but  some 


Fig.  218.    Leaves  showing  pinnate  and  palmate  branching;  the  one  to  the  left  is  from 
sumach,  that  to  the  right  from  buckeye. — Caldwell. 


very  prominent  families  have  flowers  "  in  fours."  There 
are  also  dicotyledonous  families  with  flowers  "in  threes," 
and  some  with  flowers  "  in  twos." 

It  should  be  remembered  that  no  one  of  the  above  char- 
acters, unless  it  be  the  character  of  the  embryo,  should  be 
depended  upon  absolutely  to  distinguish  these  two  groups. 


236 


PLANT  STRUCTURES 


It  is  the  combination  of  characters  which  determines  a 
group. 

Monocotyledons 

130.  Introductory. — This  great  group  gives  evidence  of 
several  distinct  lines  of  development,  distinguished  by  what 
may  be  called  the  working  out  of  different  ideas.  In  this 
way  numerous  families  have  resulted — that  is,  groups  of 


Fig.  219. 


A  leaf  of  honey  locust,  to  show  twice  pinnate  branching  (bipinnate  leaf).— 
Caldwell. 


forms  which  seem  to  belong  together  on  account  of  similar 
structures.  This  similarity  of  structure  is  taken  to  mean 
relationship.  A  family,  therefore,  is  made  up  of  a  group 
of  nearly  related  forms.  Opinions  may  differ  as  to  what 
forms  are  so  nearly  related  that  they  deserve  to  consti- 
tute a  distinct  family.  A  single  family  of  some  botanists 
may  be  "  split  up "  into  two  or  more  families  by  others. 
Despite  this  diversity  of  opinion,  most  of  the  families  are 
fairly  well  recognized. 


MONOCOTYLEDONS   AND   DICOTYLEDONS  237 

Within  a  family  there  are  smaller  groups,  indicating 
closer  relationships,  known  as  genera  (singular,  genus). 
For  example,  in  the  great  family  to  which  the  asters  belong, 
the  different  asters  resemble  one  another  more  than  they  do 
any  other  members  of  the  family,  and  hence  are  grouped 
together  in  a  genus  Aster.  In  the  same  family  the  golden- 
rods  are  grouped  together  in  the  genus  Solidago.  The 
different  kinds  of  Aster  or  of  Solidago  are  called  species 
(singular  also  species).  A  group  of  related  species,  there- 
fore, forms  a  genus  ;  and  a  group  of  related  genera  forms  a 
family. 

The  technical  name  of  a  plant  is  the  combination  of  its 
generic  and  specific  names,  the  former  always  being  written 
first.  For  example,  Quercus  alba  is  the  name  of  the  com- 
mon white  oak,  Quercus  being  the  name  of  the  genus  to 
which  all  oaks  belong,  and  alba  the  specific  name  which 
distinguishes  this  oak  from  other  oaks.  Xo  other  names 
are  necessary,  as  no  two  genera  of  plants  can  bear  the  same 
name. 

In  the  Monocotyledons  about  forty  families  are  recog- 
nized, containing  numerous  genera,  and  among  these 
genera  the  twenty  thousand  species  are  distributed.  It  is 
evident  that  it  will  be  impossible  to  consider  such  a  vast 
array  of  forms,  even  the  families  being  too  numerous  to 
mention.  A  few  important  families  will  be  mentioned, 
which  will  serve  to  illustrate  the  group. 

131.  Pondweeds. — These  are  submerged  aquatics,  found 
in  most  fresh  waters  (some  are  marine),  and  are  regarded 
as  among  the  simplest  Monocotyledons.  They  are  slender, 
branching  herbs,  growing  under  water,  but  often  having 
floating  leaves,  and  sending  the  simple  flowers  or  flower 
clusters  above  the  surface  for  pollination  and  seed-distri- 
bution. The  common  pondweed  (Potamogeton)  contains 
numerous  species  (Fig.  220),  while  Naias  (naiads)  and 
Zannichellia  (horned  pondweed)  are  common  genera  in 
ponds  and  slow  waters. 
34 


238 


PLANT  STEUCTUKES 


The  simple  character  of  these  forms  is  indicated  by  their 
aquatic  habit  and  also  by  their  flowers,  which  are  mostly 
naked  and  with  few  sporophylls.  A  flower  may  consist  of 
a  single  stamen,  or  a  single  carpel ;  or  there  may  be  several 
stamens  and  carpels  associated,  but  without  any  coalescence 
(Fig.  220,  B). 

In  the  same  general  line  with  the  pondweeds,  but  with 
more  complex  flowers,  are  the  genera   Sagittaria  (arrow- 


Fiq.  220.  Pondweed  (Potamogeton):  A,  branch  with  cluster  (spike)  of  simple  flowers, 
showing  also  the  broad  floating  leaves  and  the  narrow  submerged  ones;  B,  a  sin- 
gle flower,  showing  the  inconspicuous  perianth  lobes  (c),  the  short  stamens  (a), 
and  the  two  short  styles  with  conspicuous  stigmatic  surfaces. — A  after  Reichen- 
bach;  B  after  Le  Maout  and  Decaisnb. 


Fig.  221.  Cat-tails  {Typha\  showing  the  dense  spikes  of  very  simple  flowers,  each 
showing  two  regions,  the  lower  the  pistillate  flowers,  the  upper  the  staminate.— 
From  ""*Field,  Forest,  and  Wayside  Flowers." 


240 


PLANT  STRUCTURES 


leaf)  and  Alisma  (water-plantain),  in  which  there  is  a  dis- 
tinct calyx  and  corolla.  The  genus  Typlia  (cat-tail)  is  also 
an  aquatic  or  marsh  form  of  very  simple  type,  the  flow- 
ers being  in  dense 
cylindrical  clusters 
(spikes),  the  upper 
flowers  consisting  of 
stamens,  the  lower  of 
carpels,  thus  forming 
two  very  distinct  re- 
gions of  the  spike 
(Fig.  221). 

132.  Grasses. — 
This  is  one  of  the 
largest  and  probably 
one  of  the  most  use- 
ful groups  of  plants, 
as  well  as  one  of  the 
most  peculiar.  It  is 
world-wide  in  its  dis- 
tribution, and  is  re- 
markable in  its  dis- 
play of  individuals, 
often  growing  so 
densely  over  large 
areas  as  to  form  a 
close  turf.  If  the 
grass -like  sedges  be 
associated  with  them 
there  are  about  six 
thousand  species, 
representing  nearly 
one  third  of  the  Mon- 
ocotyledons. Here 
belong  the  various 
cereals,  sugar  canes, 


Fig.  222.  A  common  meadow  grass  (Festuca) :  A, 
portion  of  flower  cluster  (spikelet),  showing  the 
bracts,  in  the  axils  of  two  of  which  flowers  are 
exposed  ;  B,  a  single  flower  with  its  envelop- 
ing bract,  showing  three  stamens,  and  a  pistil 
whose  ovary  bears  two  style  branches  with  much 
branched  stigmas.— After  Stkasburger. 


MONOCOTYLEDONS   AND   DICOTYLEDONS  241 

bamboos,  and  pasture  grasses,  all  of  them  immensely  use- 
ful plants. 

The  flowers  are  very  simple,  having  no  evident  perianth 
(Fig.  222).  Most  commonly  a  flower  consists  of  three  sta- 
mens, surrounding  a  single  carpel,  whose  ovary  ripens  into 
the  grain,  the  characteristic  seed-like  fruit  of  the  group. 
The  stamens,  however,  may  be  of  any  number  from  one  to 
six.  The  flowers,  therefore,  are  naked,  with  indefinite  num- 
bers, and  hypogynous,  indicating  a  comparatively  simple 
type.    It  is  also  noteworthy  that  the  group  is  anemophilous. 

One  of  the  noteworthy  features  of  the  group  is  the 
prominent  development  of  peculiar  leaves  (bracts)  in  con- 
nection with  the  flowers.  Each  flower  is  completely  pro- 
tected or  even  inclosed  by  one  of  these  bracts,  and  as  the 
bracts  usually  overlap  one  another  the  flowers  are  invisible 
until  the  bracts  spread  apart  and  permit  the  long  dangling 
stamens  to  show  themselves.  These  bracts  form  the  so- 
called  "chaff"  of  wheat  and  other  cereals,  where  they 
persist  and  more  or  less  envelop  the  grain  (ripened  ovary). 
As  they  are  usually  called  glumes,  the  grasses  and  sedges 
are  said  to  be  glumaceous  plants. 

Grasses  are  not  always  lowly  plants,  for  in  the  tropics 
the  bamboos  and  canes  form  growths  that  may  well  be 
called  forests.  The  grasses  constitute  the  family  Graminece, 
and  the  sedges  the  family  Cyperacece. 

133.  Palms. — More  than  one  thousand  species  of  palms 
are  grouped  in  the  family  Palmacew.  These  are  the  tree 
Monocotyledons,  and  are  very  characteristic  of  the  tropics, 
only  the  palmetto  getting  as  far  north  as  our  Gulf  States. 
The  habit  of  body  is  like  that  of  tree-ferns  and  Cycads,  a 
tall  unbranched  columnar  trunk  bearing  at  its  summit  a 
crown  of  huge  leaves  which  are  pinnate  or  palmate  in  char- 
acter, and  often  splitting  so  as  to  appear  lobed  or  compound 
(Figs.  223,  224). 

The  flower  clusters  are  usually  very  large  (Fig.  223), 
and  each  cluster  at  first  is  inclosed  in  a  huge  bract,  which 


Fig.  223.  A  date  palm,  showing  the  unbranched  columnar  trunk  covered  with  old  leaf 
bases,  and  with  a  cluster  of  huge  pinnate  leaves  at  the  top,  only  the  lowest  por- 
tions of  which  are  shown  ;  two  of  the  very  heavy  fruit  clusters  are  also  shown. — 
From  "  Plant  Relations." 


MONOCOTYLEDONS   AND   DICOTYLEDONS 


243 


is  often  hard.  Usually  a  perianth  is  present,  but  with  no 
differentiation  of  calyx  and  corolla,  and  the  flower  parts  are 
quite  definitely  in  "  threes,"  so  that  the  cyclic  arrangement 
with  the  characteristic  Monocotyledon  number  appears. 


Fio. 


224.    A  fan  palm,  with  low  stem  and  crown  of  large  palmate  leaves,  which  have 
split  so  as  to  appear  palmately  branched.— From  "  Plant  Relations." 


134.  Aroids. — This  is  a  group  of  nearly  one  thousand 
species,  most  of  them  belonging  to  the  family  A  racece.  In 
our  flora  the  Indian  turnip  or  Jack-in-the-pulpit  (Arismma) 
(Fig.  225),  sweetflag  (Acorns),  and  skunk-cabbage  (Symplo- 
carpus),  may  be  taken  as  representatives ;  while  the  culti- 
vated Calla-lily  is  perhaps  even  better  known.  The  great 
display  of  aroids,  however,  is  in  the  tropics,  where  they  are 
endlessly  modified  in  form  and  structure,  and  are  erect,  or 
climbing,  or  epiphytic. 


244 


PLANT   STKUCTUKES 


The  flowers  are  usually  very  simple,  often  being  naked, 
with  two  to  nine  stamens,  and  one  to  four  carpels  (Fig. 


Fig.  235.    Jack-in-the-pulpit  (Ariscema),  showing  the  overarching  spathes;   in  one 
case  a  side  view  shows  the  naked  tip  of  the  projecting  spadix.— After  Atkinson. 


197).     They  are  inconspicuous  and  closely  set  upon  the 
lower  part  of  a  fleshy  axis,  which  is  naked  above  and  often 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


245 


modified  in  a  remarkable  way  into  a  club-shaped  or  tail-like 
often  brightly  colored  affair.  This  singular  flower-cluster 
with  its  fleshy  axis  is  called  a  spadix.  The  flowers  often 
include  but  one  sort  of  sporophyll,  and  staminate  and 
pistillate  flowers  hold  different  positions  upon  the  spadix 
(Fig.  226). 

The  spadix  is  enveloped  by  a  great  bract,  which  sur- 
rounds and  overarches  like  a  large  loose  hood,  and  is  called 
the  spathe.  The  spathe  is  exceedingly 
variable  in  form,  and  is  often  conspic- 
uously colored,  forming  in  the  Calla- 
lily  the  conspicuous  white  part,  within 
which  the  spadix  may  be  seen,  near  the 
base  of  which  the  flowers  are  found. 
In  Jack-in-the-pulpit  (Fig.  225)  it  is 
the  overarching  spathe  which  suggests 
the  "  pulpit."  The  spadix  and  spathe 
are  the  characteristic  features  of  the 
group,  and  the  spathe  is  variously 
modified  in  form,  structure,  and  color 
for  insect  pollination,  as  is  the  peri- 
anth of  other  entomophilous  groups. 

Aroids  are  further  peculiar  in  hav- 
ing broad  net-veined  leaves  of  the  Di- 
cotyledon type.  Altogether  they  form 
a  remarkably  distinct  group  of  Mon- 
ocotyledons. 

135.  Lilies.— The  lily  and  its  allies  are  usually  regarded 
as  the  typical  Monocotyledon  forms.  The  perianth  is 
fully  developed,  and  is  very  conspicuous,  either  undifferen- 
tiated or  with  distinct  calyx  and  corolla,  and  the  flower  is 
well  organized  for  insect  pollination.  The  flowers  are  either 
solitary  or  few  in  a  cluster  and  correspondingly  large,  or  in 
more  compact  clusters  and  smaller.  In  any  event,  the 
perianth  is  the  conspicuous  thing,  rather  than  spathes  or 
glumes. 


Fig.  226.  Spadix  of  an 
Arum,  with  spathe  re- 
moved, showing  cluster 
of  naked  pistillate  flow- 
ers at  base,  just  above 
a  cluster  of  staminate 
flowers,  and  the  club- 
shaped  tip  of  the  spa- 
dix.— After  Wossidlo. 


246 


PLANT  STRUCTURES 


In  the  general  lily  alliance,  composed  of  eight  or  nine 
families,  there  are  more  than  four  thousand  species,  repre- 
senting about  one  fifth  of  all  the  Monocotyledons,  and  they 
are  distributed  everywhere.  They  are  almost  all  terrestrial 
herbs,  and  are  prominently  geophilous  ("earth -lovers") — 

that  is,  they  develop 
bulbs,  rootstocks,  etc., 
which  enable  them  to 
disappear  from  above 
the  surface  during  un- 
favorable conditions 
(cold  or  drought),  and 
then  to  reappear  rap- 
idly upon  the  return 
of  favorable  conditions 
(Figs.  227,  228,  231, 
233). 

In  the  regular  lily 
family  (Liliacece)  the 
flowers  are  hypogy- 
nous  and  actinomor- 
phic  (Fig.  231),  the 
six  perianth  parts  are 
mostly  alike  and  some- 
times sympetalous  (as 
in  the  lily-of-the-val- 
ley,  hyacinth,  easter 
lily)  (Figs.  201,  229), 
the  stamens  are  usu- 
al ly  six  (two  sets), 
and  the  three  carpels  are  syncarpous  (Figs.  204,  230). 
This  is  a  higher  combination  of  floral  characters  than 
any  of  the  preceding  groups  presents.  Hypogyny  and 
actinomorphy  are  low,  but  a  conspicuous  perianth,  syn- 
carpy,  and  occasional  sympetaly  indicate  considerable  ad- 
vancement. 


Fig.  227.  Wake-robin  (Trillium),  showing  root- 
stock,  from  which  two  branches  arise,  each  bear- 
ing a  cycle  (whorl)  of  three  leaves  and  a  single 
trhnerous  flower.— After  Atkinson. 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


247 


In  the  amaryllis  family  (Amaryllidacece),  a  higher  fam- 
ily of  the  same  general  line,  represented  by  species  of  Nar- 
cissus (jonquils,  daffodils,  etc.),  Agave,  etc.,  the  flowers 
are  distinctly  epigynous. 


Fig.  228.  Star-of -Bethlehem  ( Ornilhogalum) :  a,  entire  plant  with  tuberous  base  and 
trimerous  flowers;  b,  a  single  flower;  c,  portion  of  flower  showing  relation  of 
parts,  perianth  lobes  and  stamens  arising  from  beneath  the  prominent  ovary  (by- 
pogynous);  d,  mature  fruit;  «,  section  of  the  syncarpous  ovary,  showing  the  three 
carpels  and  loculi.— After  Schimper. 

In  the  iris  family  (Iridacece),  the  most  highly  specialized 
family  of  the  lily  line,  and  represented  by  the  various  spe- 


Fig.  239.    The  Japan  lily,  showing  a  tubular  perianth,  the  parts  of  the  perianth 
distinct  above.— From  "Field,  Forest,  and  Wayside  Flowers." 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


249 


cies  of  Iris  (flags)  (Fig.  232),  Crocus,  Gladiolus  (Figs.  233, 
234),  etc.,  the  flowers  are  not  only  epigynous,  but  some  of 
them  are  zygomorphic. 
When  a  plant  has 
reached  both  epigyny 
and  zygomorphy  in  its 
flowers,  it  may  be  re- 
garded as  of  high  rank. 

136.  Orchids.— In 
number  of  species  this 
(Orchidacece)  is  the 
greatest  family  among 
the  Monocotyledons, 
the  species  being  vari- 
ously estimated  from 
six  thousand  to  ten 
thousand,  representing 
between  one  third  and 
one  half  of  all  known 
Monocotyledons.  In  display  of  individuals,  however,  the 
orchids  are  not  to  be  compared  with  the  grasses,  or  even 
with  lilies,  for  the  various  species  are  what  are  called  "rare 
plants" — that  is,  not  extensively  distributed,  and  often 
very  much  restricted.  Although  there  are  some  beautiful 
orchids  in  temperate  regions,  as  species  of  Habenaria  (rein- 
orchis)  (Fig.  235),  Pogonia,  Calopogon,  Calypso,  Cypripe- 
dium  (lady-slipper,  or  moccasin  flower)  (Fig.  236),  etc., 
by  far  the  greatest  display  and  diversity  are  in  the  tropics, 
where  many  of  them  are  brilliantly  flowered  epiphytes 
(Fig.  237). 

Orchids  are  the  most  highly  specialized  of  Monocoty- 
ledons, and  their  brilliant  coloration  and  bizarre  forms  are 
associated  with  marvelous  adaptation  for  insect  visitation 
(see  Plant  Relations,  pp.  134,  135).  The  flowers  are  epigy- 
nous and  strongly  zygomorphic.  One  of  the  petals  is  re- 
markably modified,   forming  a  conspicuous   lip   which   is 


Fig.  230.  Diagrammatic  cross-section  of  ovary 
of  Lilium  PhUadelphicum,  showing  the  three 
loculi,  in  each  of  which  are  two  ovules  (mega- 
sporangia);  -4,  ovule;  B,  integuments;  C,  nu- 
cellus  ;  D,  embryo-sac  (megaspore). — Cald- 
well. 


Fig.  231.  The  common  dog-tooth  violet,  showing  the  large  mottled  leaves  and  con- 
spicuous flowers  which  are  sent  rapidly  above  the  surface  from  the  subterranean 
bulb  (see  cut  in  the  left  lower  corner),  also  some  petals  and  stamens  and  the  pistil 
dissected  out.— From  "  Plant  Relations." 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


251 


modified  in  a  great  variety  of 
ways,  and  a  prominent,  often 
very  long,  sjnir,  in  the  bottom  of 
which  nectar  is  secreted,  which 
must  be  reached  by  the  proboscis 
of  an  insect  (Fig.  235).  The 
stamens  are  reduced  to  one  or 
two,  and  welded  with  the   style 


Fiq.  232.  Flower  of  flag  (Iris), 
showing  some  of  the  sepals 
and  petals,  one  of  the  three 
stamens,  and  the  distinctly  in- 
ferior ovary,  being  an  epigy- 
nous  flower.— After  Grat. 


Fig.  234.  Flower  cluster  of  Gla- 
diolus, showing  somewhat  zygo- 
morphic  flowers.— Caldwell. 


Fig.  233.  Gladiolus,  showing  tuberous  subter- 
ranean stem  from  which  roots  descend,  grass- 
like  leaves,  and  somewhat  zygomorphic  flow- 
ers.—After  Reichenbach. 


252 


PLANT  STRUCTURES 


and  stigma  tic  surface  into  an  indistinguishable  mass  in 
the  center  of  the  flowers.  The  pollen-grains  in  each  sac 
are  sticky  and  cohere  in  a  club-shaped  mass  {pollinium), 
which  is  pulled  out  and  carried  to  another  flower  by  the 


Fig.  235.  A  flower  of  an  orchid  (Habena- 
riar.  at  1  the  complete  flower  is  shown, 
with  three  sepals  behind  and  three  pet- 
als in  front,  the  lowest  one  of  which  has 
developed  a  long  strap-shaped  portion 
(lip)  and  a  still  longer  spur  portion,  the 
opening  to  which  is  seen  at  the  base  of 
the  strap,  and  behind  the  spur  the  long 
inferior  ovary  (epigynous  character) ; 
the  two  pollen  sacs  of  the  single  stamen 
are  seen  in  the  center  of  the  flower,  di- 
verging downward,  and  between  them 
stretches  the  stigma  surface  ;  the  rela- 
tion between  pollen  sacs  and  stigma  sur- 
face is  shown  in  2 ;  within  each  pollen 
sac  is  a  mass  of  sticky  pollen  (pollini- 
um),  ending   below    in   a   sticky  disk, 

which  may  be  seen  in  1  and  2 ;  in  3  a  pollen  mass  (a)  is  shown  sticking  to  each 

eye  of  a  moth.— After  Gray. 

visiting  insect.  The  whole  structure  indicates  a  very 
highly  specialized  type,  elaborately  organized  for  insect 
pollination. 

Another  interesting  epigynous  and  zygomorphic  trop- 
ical group,  but  not  so  elaborate  as  the  orchids,  is  repre- 
sented by  the  cannas  and  bananas  (Fig.  120),  common  in 
cultivation  as  foliage  plants,  and  the  aromatic  gingers. 

From  the  simple  pondweeds  to  the  complex  orchids  the 
evolution  of  the  Monocotyledons  has  proceeded,  and  be- 
tween them  many  prominent  and  successful  families  have 
been  worked  out. 


Fig.  236.    A  clump  of  lady-slippers  (Cypripedium),  showing  the  habic  of  the  plant 
and  the  general  structure  of  the  zygomorphic  flower.— After  Gibson. 
35 


254 


PLANT   STRUCTURES 


Fig.  237.    A  group  of  orchids  ( Cattleya),  showing  the  very  zygomorphic  flowers,  the 
lip  being  well  shown  in  the  flower  to  the  left  (lowest  petal).— Caldwell. 


Dicotyledons 

137.  Introductory. — Dicotyledons  form  the  greatest  group 
of  plants  in  rank  and  in  numbers,  being  the  most  highly 
organized,  and  containing  about  eighty  thousand  species. 
They  represent  the  dominant  and  successful  vegetation  in 
all  regions,  and  are  especially  in  the  preponderance  in  tem- 
perate regions.  They  are  herbs,  shrubs,  and  trees,  of  every 
variety  of  size  and  habit,  and  the  rich  display  of  leaf  forms 
is  notably  conspicuous. 

Two  great  groups  of  Dicotyledons  are  recognized,  the 
Archichlamydece  and  the  Sympetalce.  In  the  former  there 
is  either  no  perianth  or  its  parts  are  separate  (polypeta- 
lous) ;  in  the  latter  the  corolla  is  sympetalous.  The  Archi- 
chlamydeae  are  the  simpler  forms,  beginning  in  as  simple  a 
fashion  as  do  the  Monocotyledons ;  while  the  Sympetalas 


MONOCOTYLEDONS   AND    DICOTYLEDONS 


255 


are  evidently  derived  from  them  and  become  the  most 
highly  organized  of  all  plants.  The  two  groups  each  con- 
tain about  forty  thousand  species,  but  the  Archichlamydese 
contain  about  one  hundred  and  sixty  families,  and  the 
Sympetalse  about  fifty. 

To  present  over  two  hundred  families,  containing  about 
eighty  thousand  species,  is  clearly  impossible,  and  a  very 
few  of  the  prominent  ones  will  be  selected  for  illustrations. 


Archichlamydem 

138.  Poplars  and  their  allies. — This  great  alliance  repre- 
sents nearly  five  thousand  species,  and  seems  to  form  an 
isolated  group.  It  is  a  notable  tree  assemblage,  and  appar- 
ently the  most  primitive  and  ancient  group  of  Dicotyledons, 
containing  the  most  important  deciduous  forest  forms  of 


Fig.  238.    An  oak  in  winter  condition.— From  "  Plant  Kelations." 


256 


TLANT   STRUCTURES 


temperate  regions,  for  here  belong  the  oak  (Fig.  238),  hick« 
ory,  walnut,  chestnut,  beech,  poplar,  birch,  elm  (Figs.  198, 
239),  willow  (Fig.  240),  etc.  The  primitive  character  is  in- 
dicated not  merely  by  the  floral  structures,  but  also  by  the 
general  anemophilous  habit. 

In  the  poplar  (Populus)  and  its  allied  form,  the  willow 
(Salix),  the  flowers  are  naked  and  hypogynous  (Fig.  196), 


Fig.  239.    An  elm  in  foliage.— From  "Plant  Relations." 


MONOCOTYLEDONS   AND    DICOTYLEDONS 


257 


the  stamens  are  indefinite  in  number  (two  to  thirty),  and 
the  pistil  is  syncarpous  (two  carpels).     The  stamens  and 


Fig.  240.    Flower  clusters  of  willow  (amenta);  that  to  the  left  is  pistillate,  the  other 
staminate.— After  Warming. 

pistils  are  not  only  separated  in  different  flowers,  but  upon 
different  plants,  some  plants  being  staminate  and  others 
pistillate  (Fig.  240).  The  flowers  are  clustered  upon  a  long 
axis,  and  each  one  is 
protected  by  a  promi- 
nent bract.  It  is  these 
scaly  bracts  which 
give  character  to  the 
cluster,  which  is  called 
an  ament  or  catkin, 
and  the  plants  which 
produce  such  clusters 
are  said  to  be  amenta- 
ceous. These  aments 
of  poplars,  "pussy 
willows,"  and  the 
alders  and  birches  are 
very  familiar  objects 
(Figs.  240,  241). 


Fig.  241.  Aments  of  alder  (Alnus) :  a,  branch 
with  staminate  aments  (n),  pistillate  aments 
(«i),  and  a  young  bud  (£);  b,  pistillate  ament  at 
time  of  discharging  seeds,  showing  the  promi- 
nent bracts.— After  Warming. 


258 


PLANT   STRUCTURES 


The  only  advanced  character  in  the  flowers  as  described 
above  is  the  syncarpous  pistil,  but  in  the  great  allied  pepper 
family  (Piperacece)  of  the  tropics,  with  its  one  thousand 
species,  and  most  nearly  represented  in  our  flora  by  the 


Pig.  242.  Ovule  of  hornbeam  (Carpimis),  showing  chalazogamy:  m,  the  micropyle; 
pt,  the  pollen  tube,  which  may  be  traced  to  its  entrance  into  the  embryo-sac  at  its 
antipodal  end,  and  thence  upward  through  the  sac  toward  the  egg.— After  Mart 
Ewabt. 


lizard-tail  (Saururus)  of  the  swamps  (Fig.  195),  the  flowers 
are  not  merely  naked,  but  also  apocarpous,  and  the  whole 
structure  is  much  like  that  of  the  simplest  Monocotyle- 


MONOCOTYLEDONS   AND   DICOTYLEDONS  259 

dons.  The  peppers  seem  to  represent  the  simplest  of  the 
Dicotyledons,  and  this  great  line  may  have  begun  with 
some  such  forms. 

A  very  interesting  fact  in  connection  with  the  fertiliza- 
tion of  certain  amentaceous  plants  has  been  discovered. 
In  birch,  alder,  walnut,  hornbeam,  and  some  others,  the 
pollen-tube  does  not  eiiter  the  ovule  by  way  of  the  micro- 
pyle,  but  pierces  through  in  the  region  of  the  base  of  the 
ovule  and  so  penetrates  to  the  embryo-sac  (Fig.  242).  As 
the  region  of  the  ovule  where  integument  and  nucellus  are 
not  distinguishable  is  called  the  chalaza,  this  phenomenon 
is  known  as  chalazogamy,  meaning  "fertilization  through 
the  chalaza." 

139.  Buttercups  and  their  allies. — This  is  a  great  assem- 
blage of  terrestrial  herbs,  including  nearly  five  thousand 
species,  and  is  thought  by  many  to  be  the  great  stock  from 
which  most  of  the  higher  Dicotyledons  have  been  derived. 
The  alliance  includes  the  water-lilies,  buttercups,  and  pop- 
pies, the  specialized  mustards,  and  certain  notable  tree 
forms,  as  magnolias,  custard-apples,  and  the  tropical  laurels 
with  one  thousand  species  represented  in  our  flora  only 
by  the  sassafras.  Here  also  is  the  strange  group  of  "  car- 
nivprous"  plants  (Sarracenia,  Drosera,  Dionma,  etc.).  The 
group  is  distinctly  entomophilous,  in  striking  contrast  with 
the  preceding  one. 

Taking  the  buttercup  {Ranunculus)  as  a  type  (Fig.  202), 
the  flower  is  hypogynous,  the  calyx  and  the  corolla  are  dis- 
tinctly differentiated  and  actinomorphic,  and  adapted  for 
insect-pollination,  but  the  spiral  arrangement  and  indefinite 
numbers  are  very  apparent,  notably  in  connection  with  the 
apocarpous  pistils,  which  are  very  numerous  upon  a  promi- 
nent receptacle,  but  involving  more  or  less  all  the  parts. 
The  stamens  are  also  very  numerous  (Figs.  200,  243,  244). 
In  the  water-lilies  the  petals  and  stamens  are  indefinitely 
numerous  (Fig.  203),  and  in  the  poppies  there  is  no  definite 
number.     In  many  of  the  forms,  however,  in  connection 


Pig.  343.  Marsh  marigold  (Caltha),  a  member  of  the  Buttercup  family,  also  showing 
floral  diagram,  in  which  the  floral  leaves  are  five,  but  the  stamens  and  apocarpous 
pistils  are  indefinitely  numerous.— After  Atkinson. 


Fig.  244.  Zygomorphic  flower  of  larkspur 
(Delphinium),  with  sepals  removed,  show- 
ing two  petals  with  prominent  spurs,  and 
numerous  stamens. — After  Baillon. 


Fig.  245.  Diagram  of  the  zygomorphic 
flower  of  larkspur  (Delphinium),  show- 
ing the  spur  developed  by  a  sepal  and 
inclosing  the  two  petal  spurs. — After 
Baillon. 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


261 


with  one  or  more  of  the  parts,  the  Dicotyl  number  (five) 
appears  (Figs.  243,  245),  but  with  no  special  constancy. 

In  certain  genera  of  the  buttercup  family  (Ranuncula- 
cecp)  zygomorphy  appears,  as  in  the  larkspur  (Delphinium) 
with  its  spurred  petals  and  sepals  (Figs.  244,  245),  and  the 
monkshood  (Aconitum)  with  its  hooded  sepal ;  and  in  the 


Fig.  246.  The  common  cabbage  (Brassica),  a  member  of  the  mustard  family:  A, 
flower  cluster,  showing  buds  at  tip,  open  flowers  below  with  four  spreading  petals, 
and  forming  pods  below ;  B.  mature  pod,  with  the  persistent  style;  C,  pod  opening 
by  two  valves,  and  showing  seeds  attached  to  the  false  partition.— After  Warming. 

water-lily  family  (Nymplmacem)  and  poppy  family  (Pajxi- 
veracece)  syncarpy  appears.  In  this  alliance,  also,  belong 
the  sweet-scented  shrubs  (Calycanthus),  with  their  perigy- 
nous  flowers  containing  numerous  parts  (Fig.  206). 


262 


PLANT   STRUCTURES 


Fig.  347.  Diagram  of  crucifer 
flower,  showing  the  relations 
of  parts  ;  four  sepals,  four 
petals,  six  stamens,  and  one 
carpel  with  a  false  partition. 
—After  Warming. 


The  most  specialized  large  group  in  this  alliance  is 
the  mustard  family  (Cruciferce) ,  with  twelve  hundred 
species,  to  which  belong  the  mustards,  cresses,  shep- 
herd's purse,  peppergrass,  radish,  cabbage  (Fig.  246),  etc. 
The  sepals  are  four  in  two  sets,  the 
petals  four  in  one  set,  the  stamens 
six  with  two  short  ones  in  an  outer 
set  and  four  long  ones  in  an  inner 
set,  and  one  pistil  whose  ovary  be- 
comes divided  into  two  loculi  by 
what  is  called  a  "false  partition" 
(Figs.  246,  C,  247),  and  usually  be- 
comes an  elongated  pod  (Fig.  246, 
Ay  B).  This  specialized  structure 
of  the  flower  distinctly  marks  the 
family,  whose  name  is  suggested 
by  the  fact  that  the  four  spreading 
petals  often  form  a  Maltese  cross  (Fig.  246,  A).  The  pecul- 
iar stamen  character,  four  long  and  two  short  stamens,  is 
called  tetradynamous  ("four  strong"). 

140.  Roses. — This  family  (Rosacem)  of  one  thousand 
species  is  one  of  the  best  known  and  most  useful  groups  of 
the  temperate  regions.  In  it  are  such  forms  as  Spircea, 
five-finger  (Poten- 
tilla),  strawberry 
(Fragaria)  (Figs. 
191,  207),  raspberry 
(Fig.  248),  and 
blackberry  [Bu- 
bus),  rose  (Rosa), 
hawthorn  (CratcB- 
gus),  apple,  and 
pear  (Pirus)  (Fig. 
249),  plum,  cherry, 
almond,  and  peach 
(Prunus). 


Fig.  248.  The  common  raspberry:  the  figure  to  the 
left  showing  flower-stalk,  calyx,  old  stamens 
(«),  and  prominent  receptacle,  from  which  the 
"fruit"  (a  cluster  of  small  stone  fruits,  each 
representing  a  carpel)  has  been  removed.— After 
Bailey. 


MONOCOTYLEDONS   AND   DICOTYLEDONS 


2G3 


Many  of  the  true  roses  have  a  strong  resemblance  (Fig. 
207)  to  the  buttercups  (Ranunculus),  with  their  hypogy- 
nous  regular  flowers,  and  indefinite  number  of  stamens  and 
carpels,  but  the  sepals  and  petals  are  much  more  frequently 
five,  the  Dicotyl  number  being  better  established.      The 


Fig.  249.  The  common  pear  (Pints  communis),  showing  hraneh  with  flowers  (1).  sec- 
tion of  a  flower  (x?)  showing  its  epigynous  character,  section  of  fruit  i.n  showing 
the  thickened  calyx  outside  of  the  ovary  or  "core"  (indicated  by  dotted  outline}, 
and  flower  diagram  (i)  showing  all  the  organs  in  fives  except  the  stamens.— After 
Wossidlo. 

whole  family  remains  actinomorphic,  but  perigyny  and 
epigyny  appear  in  certain  forms  (Fig.  205),  giving  rise  to 
the  peculiar  fruit  (pome)  of  apples  and  pears  (Fig.  249),  in 
which  the  calyx  and  ovary  ripen  together.  Another  spe- 
cialized group  of  roses  is  that  which  develops  the  stone- 


264 


PLANT   STRUCTURES 


fruits   (drupes),   as  apricots,   peaches   (Fig.  189),   plums, 
cherries. 

141.  Legumes. — This  is  far  the  greatest  family  (Legumi- 
nosce)  of  the  Archichlamydeae,  containing  about  seven  thou- 
sand species,  distributed  everywhere  and  of  every  habit.  It 
is  the  great  zygomorphic  group  of  the  Archichlamydeas, 
being  elaborately  adapted  to  insect  pollination.    The  more 


Fig.  250.  A  legume  plant  (Lotus),  showing  flowering  branch  (1),  a  single  flower  (2) 
showing  zygomorphic  corolla,  the  cluster  of  ten  stamens  (8)  which  with  the  carpel 
is  included  in  the  keel,  the  solitary  carpel  (h)  which  develops  into  the  pod  or  le- 
gume (5),  the  petals  (6)  dissected  apart  and  showing  standard  (a),  wings  (b),  and 
the  two  lower  petals  (c)  which  fold  together  to  form  the  keel,  and  the  floral  dia- 
gram (7).— After  Wossidlo. 


primitive  forms  of  the  Leguminosge,  the  mimosas,  acacias 
(Fig.  251),  etc.,  very  much  resemble  true  roses  and  the  but- 
tercups, with  their  hypogynous  regular  flowers  and  nu- 
merous stamens,  but  the  vast  majority  are  Papilio  forms 
with  very  irregular  (zygomorphic)  flowers  and  few  stamens 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


265 


(Fig.  250).  The  petals  are  very  dissimilar,  the  upper  one 
(standard)  being  the  largest,  and  erect  or  spreading,  the  two 
lateral  ones  (icings)  oblique  and  descending,  the  two  lower 
ones  coherent  by  their  edges  to  form  a  projecting  boat-shaped 
body  (heel),  which 
incloses  the  sta- 
mens and  pistil. 
From  a  fancied  re- 
semblance to  a  but- 
terfly such  flowers 
are  said  to  be  papil- 
ionaceous. 

The  whole  fam- 
ily is  further  char- 
acterized by  the  sin- 
gle carpel,  which 
after  fertilization 
develops  a  pod 
(Fig.  250, 5),  which 
often  becomes  re- 
markably large  as 
compared  with  the 
carpel.  It  is  this 
peculiar  pod  (le- 
gume) which  has 
given  to  the  family 
its  technical  name 
Leguminosce  and 
the  common  name 
"Legumes." 

Well-known  members  of  the  family  are  lupine  ( Lupi- 
nus),  clover  (Trifolium),  locust  (Robinia),  Wistaria,  pea 
(Pisum),  bean  (Phaseolus),  tragacanth  (Astragalus),  vetch 
(Vicia),  redbud  (Cercis),  senna  (Cassia),  honey-locust 
(Gleditschia),  indigo  (Indigo/era),  sensitive-plants  (Acacia, 
Mimosa,  etc.)  (Fig.  251),  etc. 


Fig.  251.  A  sensitive-plant  (Acacia),  showing  the 
flowers  with  inconspicuous  petals  and  very  nu- 
merous stamens,  and  the  pinnately  branched  sen- 
sitive leaves. — After  Meter  and  Schumann. 


266 


PLANT   STRUCTURES 


142.  Umbellifers. — This  is  the  most  highly  organized 
family  ( Umbelliferce)  of  the  Archichlamydeae,  which  may 
be  said  to  extend  from  Peppers  to  Umbellifers.  The  Le- 
gumes adopt  zygomorphy,  but  remain  hypogynous  ;  and  in 
some  of  the  Koses  epigyny  appears ;  but  the  Umbellifers 
with  their  fifteen  hundred  species  are  all  distinctly  epigy- 


Fig.  252.  The  common  carrot  (Dauciis  Carota):  A.  branch  bearing  the  compound 
umbels;  B,  a  single  epigynous  flower,  showing  inferior  ovary,  five  spreading 
petals,  five  stamens  alternating  with  the  petals,  and  the  two  styles  of  the  bicarpel- 
lary  pistil;  C,  section  of  flower,  showing  relation  of  parts,  and  also  the  minute 
sepals  near  the  top  of  the  ovary  and  just  beneath  the  other  parts. — After  Warming. 


nous  (Fig.  252,  B,  C),  being  one  of  the  very  few  epigy- 
nous  families  among  the  Archichlamydeae.  In  addition 
to  epigyny,  the  cyclic  arrangement  and  definite  Dicotyl 
number  is  established,  there  being  five  sepals,  five  petals, 
five  stamens,  and  two  carpels,  the  highest  known  floral 


MONOCOTYLEDONS   AND   DICOTYLEDONS 


267 


formula,  and  one  that  appears  among  the  highest  Sym- 
petalae. 

The  name  of  the  family  is  suggested  by  the  character- 
istic inflorescence,  which  is  also  of  advanced  type.     The 
flowers    are    reduced    in 
size  and  massed  in  flat-         /0§?fii 
topped  clusters  called 
umbels  (Figs.  252,  A,  253). 
The  branches  of  the  clus- 
ter arise   in   cycles  from 
the  axis  like  the  braces 
of  an  umbrella.     As  a  re- 
sult of  the  close  approxi- 
mation of  the  flowers  the 
sepals  are  much  reduced 
in  size  and  often  obsolete 
(Fig.  252,  G). 

The  Umbellifers  are 
mainly  perennial  herbs  of 
the  north  temperate  re- 
gions, forming  a  very  dis- 
tinct family,  and  contain- 
ing the  following  familiar 
forms :  carrot  (Daucus) 
(Fig.  252),  parsnip  (Pasti- 
naca),  hemlock  (Conium) 
(Fig.  253),  pepper-and- 
salt  (Erigenia),  caraway 
(Carum),  fennel  (Fcenic- 
ulum),  coriander  (Cori- 
andrum),  celery  (Api- 
um),  parsley  {Petroseli- 
num),  etc.  Allied  to  the 
Umbellifers  are  the  Ara- 
lias  (Araliacece),  and  the 
Dogwoods  (Cornacece). 


Fig.  253.  Hemlock  ( Conium),  an  Umbellifer, 
showing  the  umbels,  with  the  principal 
rays  rising  from  a  cycle  of  bracts  (invo- 
lucre), and  each  bearing  at  its  summit  a 
secondary  umbel  with  its  cycle  of  second- 
ary bracts  (involucel).— After  Schimpeb. 


268  PLANT  STRUCTURES 

Sympetalm 

143.  Introductory. — These  are  the  highest  and  the  most 
recent  Dicotyledons.  While  they  contain  numerous  shrubs 
and  trees  in  the  tropics,  they  are  by  no  means  such  a  shrub 
and  tree  group  in  the  temperate  regions  as  are  the  Archi- 
chlamydeae.  The  flowers  are  constantly  cyclic,  the  num- 
ber five  or  four  is  established,  and  the  corolla  is  sympeta- 
lous, the  stamens  usually  being  borne  upon  its  tube  (Figs. 
208,  209,  212). 

There  are  two  well-defined  groups  of  Sympetalae,  distin- 
guished from  one  another  by  the  number  of  cycles  and  the 
number  of  carpels  in  the  flower.  The  group  containing 
the  lower  forms  is  pentacyclic,  meaning  "  cycles  five,"  there 
being  two  sets  of  stamens.  In  it  also  there  are  five  carpels, 
the  floral  formula  being,  Sepals  5,  Petals  5,  Stamens  5  +  5, 
Carpels  5.  As  the  carpels  are  the  same  in  number  as  the 
other  parts,  the  flowers  are  called  isocarpic,  meaning  "  car- 
pels same."  The  group  is  named  either  Pentacyclai  or  Iso- 
carpce,  and  contains  about  ten  families  and  4,000  species. 

The  higher  groups,  containing  about  forty  families  and 
36,000  species,  is  tetracyclic,  meaning  "  cycles  four,"  and 
anisocarpic,  meaning  "carpels  not  the  same,"  the  floral 
formula  being,  Sepals  5,  Petals  5,  Stamens  5,  Carpels  2. 
The  group  name,  therefore,  is  Tetracyclce  or  Anisocarpce. 

144.  Heaths.  —The  Heath  family  {Ericacece)  and  its  allies 
represent  about  two  thousand  species.  They  are  mostly 
shrubs,  sometimes  trailing,  and  are  displayed  chiefly  in 
temperate  and  arctic  or  alpine  regions,  in  cold  and  damp 
or  dry  places,  often  being  prominent  vegetation  in  bogs 
and  heaths,  to  which  latter  they  give  name  (Fig.  254).  The 
flowers  are  pentacyclic  and  isocarpic,  as  well  as  mostly  hyp- 
ogynous  and  actinomorphic.  It  is  interesting  to  note  that 
some  forms  are  not  sympetalous,  the  petals  being  distinct, 
showing  a  close  relationship  to  the  Archichlamydeae.  One 
of  the  marked  characteristics  of  the  group  is  the  dehiscence 


MONOCOTYLEDONS   AND    DICOTYLEDONS  269 

of  the  pollen-sacs  by  terminal  pores,  which  are  often  pro- 
longed into  tubes  (Fig.  255). 


Fig.  254.  Characteristic  heath  plants:  A,  B,  C,  Lyonia,  showing  sympetalous  flowers 
and  single  style  from  the  lobed  syncarpous  ovary;  D,  two  forms  of  Casdope, 
showing  trailing  habit,  small  overlapping  leaves,  and  sympetalous  flowers,  but  in 
the  smaller  form  the  petals  are  almost  distinct. — After  Drude. 

Common  representatives  of  the  family  are  as  follows : 
huckleberry  {Gaylussacia) ,  cranberry  and  blueberry  (  Vac- 
cinium),  bearberry  (Arctostaphylos),  trailing  arbutus  {Epi- 
36 


270 


PLANT   STRUCTURES 


gcea),  wintergreen  (Gaultheria),  heather  (Calluna),  moun- 
tain laurel  (Kalmia),  Azalea,  Rhododendron  (Fig.  256), 
Indian  pipe  (Monotropa),  etc. 


Fig.  255.  Flowers  of  heath  plants  (Erica),  showing  complete  flowers  (.4),  the  sta- 
mens with  "two-horned"  anthers  which  discharge  pollen  throngh  terminal  pores, 
and  the  lobed  syncarpous  ovary  with  single  style  and  prominent  terminal  stigma 
(B,  C,  D).—  After  Drude. 


145.  Convolvulus  forms. — The  well-known  morning-glory 
(Ipomcea)  (Fig.  209)  may  be  taken  as  a  type  of  the  Convol- 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


271 


vulus  family  (Convolvulacece).  Allied  with  it  are  Polemo- 
nium  and  Phlox  (Fig.  210,  b)  (Polemomacew),  the  gentians 
(Ge)itianacece),  and  the  dog-banes  (Apocynacem)  (Fig.  257). 
It  is  here  that  the  regular  sympetalous  flower  reaches  its 
highest  expression  in  the  form  of  conspicuous  tubes,  f un- 


Fig.  256.    A  cluster  of  Rhododendron  flowers.— After  Hooker. 

nels  (Fig.  258),  trumpets,  etc.  The  flowers  are  tetracyclic 
and  anisocarpic,  besides  being  hypogynous  and  actinomor- 
phic.  These  regular  tubular  forms  represent  about  five 
thousand  species,  and  contain  many  of  the  best-known 
flowers. 


272 


PLANT  STRUCTURES 


146.  Labiates. — This  great  family  {Labiatce)  and  its  alli- 
ances represent  more  than  ten  thousand  species.     Tne  con- 
spicuous   feature    is    the 
zygomorphic    flower,    dif- 
fering in  this  regard  from 
the     Convolvulus     forms, 
which    they    resemble 
being  tetracyclic  and  ani- 
socarpic,  as  well  as  hypogy- 
nous.      The    irregularity 
consists  in  organizing  the 
mouth  of  the  sympetalous 
corolla   into   two    "lips," 
resulting  in  the  labiate  or 


Fig.  257.    A  common  dogbane  (Apocynum). — From  "Field,  Forest,  and  Wayside 

Flowers." 


Fig  258     The  hedge  bindweed  (  Con  wlvulm  I,  showing  the  twining  habit  and  the  con- 
spicuous funnelform  corollas.-From  "  Field,  Forest,  and  Wayside  Flowers. 


274 


PLANT   STRUCTURES 


bilabiate  structure  (Fig.  210,  c,  d,  e),  and  suggesting  the 
name  of  the  dominant  family.  The  upper  lip  usually  con- 
tains two  petals,  and  the  lower  three  ;  the  two  lips  are  some- 
times widely  separated,  and  sometimes  in  close  contact,  and 
differ  widely  in  relative  prominence. 

Associated  with  zygomorphy  in  this  group  is  a  frequent 
reduction  in  the  number  of  stamens,  which  are  often  four 
(Fig.  212)  or  two.  The  whole  structure  is  highly  special- 
ized for  the  visits  of  insects,  and  this  great  zygomorphic 
alliance  holds  the  same 
relative  position  among 
Sympetalae  as  is  held 
by  the  zygomorphic  Le- 
gumes among  Archi- 
chlamydeae. 

In  the  mint  family, 
as  the  Labiates  are  often 
called,  there  are  about 
two  thousand  seven  hun- 
dred species,  including 
mint  {Mentha)  (Fig. 
212),  dittany  (Cunila), 
hyssop  {Hyssopus),  mar- 
joram    {Origanum), 


Fig.  259.  Flowers  of  dead  nettle  (La- 
mium) :  A,  entire  bilabiate  flower ; 
B,  section  of  flower,  showing  rela- 
tion of  parts.— After  Warming. 


Fig.  260.  A  labiate  plant  (Teucrium),  show- 
ing branch  with  flower  clusters  (^4),  and 
side  view  of  a  few  flowers  (B),  showing 
their  bilabiate  character. — After  Briquet. 


MONOCOTYLEDONS  AND  DICOTYLEDONS      275 

thyme  {Thymus),  balm  {Melissa),  sage  {Salvia),  catnip 
(Nepeta),  skullcap  {Scutellaria),  horehound  {Marrubium), 
lavender  {Lavandula),  rosemary  {Rosmarinus),  dead  nettle 
{Lamium)  (Fig.  259),  Teucrium  (Figs.  213,  260),  etc.,  a 
remarkable  series  of  aromatic  forms. 

Allied  is  the  Nightshade  family  {Solanaceai),  with  fif- 
teen hundred  species,  containing  such  common  forms  as 
the  nightshades  and  potato  {Solatium),  tomato  {Lycoper- 
sicum),  tobacco  {Nicotiana)  (Fig.  208),  etc.,  in  which  the 
corolla  is  actinomorphic  or  nearly  so ;  also  the  great  Fig- 
wort  family  {Scrophulariacece) ,  with  two  thousand  species, 
represented  by  mullein  ( Verbascum),  snapdragon  {Antir- 
rhinum) (Fig.  210,  e),  toad-flax  {Linaria)  (Fig.  210,  d), 
Pentstemon,  speedwell  ( Veronica),  Gerardia,  painted  cup 
{Castilleia),  etc.;  also  the  Verbena  family  ( Verbenacece), 
with  over  seven  hundred  species ;  and  the  two  hundred 
plantains  {Plant aginacem),  etc. 

147.  Composites. — This  greatest  and  ranking  family 
( Compositce)  of  Angiosperms  is  estimated  to  contain  at  least 
twelve  thousand  species,  containing  more  than  one  seventh 
of  all  known  Dicotyledons  and  more  than  one  tenth  of  all 
Seed-plants.  Not  only  is  it  the  greatest  family,  but  it  is 
the  youngest.  Composites  are  distributed  everywhere,  but 
are  most  numerous  in  temperate  regions,  and  are  mostly 
herbs. 

The  name  of  the  family  suggests  the  most  conspicuous 
feature — namely,  the  remarkably  complete  organization  of 
the  numerous  small  flowers  into  a  compact  head  which 
resembles  a  single  flower,  formerly  called  a  "compound 
flower."  Taking  the  head  of  an  Arnica  as  a  type  (Fig. 
261),  the  outermost  set  of  organs  consists  of  more  or  less 
leaf -like  bracts  or  scales  {involucre),  which  resemble  sepals  ; 
within  these  is  a  circle  of  flowers  with  conspicuous  yellow 
corollas  {rays),  which  are  zygomorphic,  being  split  above 
the  tubular  base  and  flattened  into  a  strap-shaped  body, 
and  much  resembling  petals  (Fig.  261,  A,  D) ;  within  the 


Fig.  261.  Flowers  of  Arnica:  .4,  lower  part  of  stem,  and  upper  part  bearing  a 
head,  in  which  are  seen  the  conspicuous  rays  and  the  disk:  I),  Bingle  ray  flower, 
showing  the  corolla,  tubular  at  base  and  strap-shaped  above,  the  two-parted  style, 
the  tuft  of  pappus  hairs,  aud  the  inferior  ovary  which  develops  into  a  seed-like 
fruit  (akene);  E.  single  disk  flower,  showing  tubular  corolla  with  spreading  limb, 
the  two-parted  style  emerging  from  the  top  of  the  Stamen  tube,  the  prominent 
pappus,  and  the  inferior  ovary  or  akene;  C,  a  single  stamen. — After  Hoffman. 
276 


MONOCOTYLEDONS  AND   DICOTYLEDONS 


277 


ray-flowers  is  the  broad  expanse  supplied  by  a  very  much 
broadened  axis,  and  known  as  the  disk  (Fig.  261,  A),  which 
is  closely  packed  with  very  numerous  small  and  regular 
tubular  flowers,  known  as  dish-flowers  (Fig.  261,  e). 


Fig.  262.  The  common  dandelion  ( Taraxacum):  1,  two  flower  stalks;  in  one  the  head 
is  closed,  showing  the  double  involucre,  the  inner  erect,  the  outer  reflexed,  in  the 
other  the  head  open,  showing  that  all  the  flowers  are  strap-shaped;  S,  a  single 
flower  shewing  inferior  ovary,  pappus,  corolla,  stamen  tube,  and  two-parted  style; 
;;,  a  mature  akene;  4,  a  head  from  which  all  but  one  of  the  akenes  have  been  re- 
moved, showing  the  pitted  receptacle  and  the  prominent  pappus  beak.— After 
Strasburuer. 

The  division  of  labor  among  the  flowers  of  a  single  head 
is  plainly  marked,  and  sometimes  it  becomes  quite  com- 
plex. The  closely  packed  flowers  have  resulted  in  modify- 
ing the  sepals  extremely.     Sometimes  they  disappear  en- 


278 


PLANT   STKUCTUKES 


tirely ;  sometimes  they  become  a  tuft  of  delicate  hairs,  as 
in  Arnica  (Fig.  261,  D,  E),  thistle  (Cnicus),  and  dandelion 
(Taraxacum)  (Fig.  263),  surmounting  the  seed-like  akene 
and  aiding  in  its  transportation  through  the  air  ;  sometimes 
they  are  converted  into  two  or  more  tooth-like  and  often 


Fig.  263.  Flowers  of  dandelion,  showing  action  of  style  in  removing  pollen  from  the 
stamen  tube:  /,  style  having  elongated  through  the  tube  and  carrying  pollen;  2, 
style  branches  beginning  to  recurve;  3,  style  branches  completely  recurved.-^ 
From  "Field,  Forest,  and  Wayside  Flowers." 


barbed  processes  arising  from  the  akene,  as  in  tickseed 
(Coreopsis)  and  beggar-ticks  (Fig.  188)  or  Spanish  needles 
(Bidens),  to  lay  hold  of  passing  animals  ;  sometimes  they 
become  beautifully  plumose  bristles,  as  in  the  blazing  star 
(Liatris)  ;  sometimes  they  simply  form  a  more  or  less  con- 
spicuous cup  or  set  of  scales  crowning  the  akene.  In  all 
of  these  modifications  the  calyx  is  called  pappus. 

The  stamens  within  the  corolla  are  organized  into  a 
tube  by  their  coalescent  anthers  (Fig.  263),  and  discharge 
their  pollen  within,  which  is  carried  to  the  surface  of  the 


MONOCOTYLEDONS   AND   DICOTYLEDONS  279 

head  and  exposed  by  the  swab-like  rising  of  the  style  (Fig. 
263).  The  head  is  thus  smeared  with  pollen,  and  visiting 
insects  can  not  fail  to  distribute  it  over  the  head  or  carry 
it  to  some  other  head. 

In  the  dandelion  and  its  allies  the  flowers  of  the  disk 
are  like  the  ray-flowers,  the  corolla  being  zygomorphic  and 
strap-shaped  (Figs.  262,  263). 

The  combination  of  characters  is  sympetalous,  tetracyc- 
lic, and  anisocarpic  flowers,  which  are  epigynous  and  often 
zygomorphic,  with  stamens  organized  into  a  tube  and  calyx 
modified  into  a  pappus,  and  numerous  flowers  organized 
into  a  compact  involucrate  head  in  which  there  is  more  or 
less  division  of  labor.  There  is  no  group  of  plants  that 
shows  such  high  organization,  and  the  Compositae  seem  to 
deserve  the  distinction  of  the  highest  family  of  the  plant 
kingdom. 

The  well-known  forms  are  too  numerous  to  mention, 
but  among  them,  in  addition  to  those  already  mentioned, 
there  are  iron-weed  ( Vernonia),  Aster,  daisy  (Bellis), 
goldenrod  {Solidago),  rosin-weed  and  compass-plant  {Silph- 
ium),  sunflower  (Helianthus),  Chrysanthemum,  ragweed 
{Ambrosia),  cocklebur  {Xanthium),  ox-eye  daisy  (Leucan- 
themum),  tansy  (Tanacetum),  wormwood  and  sage-brush 
{Artemisia),  lettuce  {Lactuca),  etc. 


CHAPTER  XV 

DIFFERENTIATION  OF  TISSUES 

148.  Introductory. — Among  the  simplest  Thallophytes 
the  cells  forming  the  body  are  practically  all  alike,  both  as 
to  form  and  work.  What  one  cell  does  all  do,  and  there 
is  very  little  dependence  of  cells  upon  one  another.  As 
plant  bodies  become  larger  this  condition  of  things  can  not 
continue,  as  all  of  the  cells  can  not  be  put  into  the  same 
relations.  In  such  a  body  certain  cells  can  be  related  to 
the  external  food  supply  only  through  other  cells,  and  the 
body  becomes  differentiated.  In  fact,  the  relating  of  cells 
to  one  another  and  to  the  external  food-supply  makes  large 
bodies  possible. 

The  first  differentiation  of  the  plant  body  is  that  which 
separates  nutritive  cells  from  reproductive  cells,  and  this  is 
accomplished  quite  completely  among  the  Thallophytes. 
The  differentiation  of  the  tissues  of  the  nutritive  body, 
however,  is  that  which  specially  concerns  us  in  this  chapter. 

A  tissue  is  an  aggregation  of  similar  cells  doing  similar 
work.  Among  the  Thallophytes  the  nutritive  body  is  prac- 
tically one  tissue,  although  in  some  of  the  larger  Thallo- 
phytes the  outer  and  the  inner  cells  differ  somewhat.  This 
primitive  tissue,  composed  of  cells  with  thin  walls  and  ac- 
tive protoplasm,  and  to  be  regarded  as  the  parent  tissue,  is 
called  parenchyma. 

Among  the  Bryophytes,  in  the  leafy  gametophore  and 

in  the  sporogonium,  there  is  often  developed  considerable 

dissimilarity  among  the  cells  forming  the  nutritive  body, 

but  the  cells  may  all  still  be  regarded  as  parenchyma.     It 

280 


DIFFERENTIATION  OF  TISSUES  281 

is  in  the  sporophyte  of  the  Pteridophytes  and  Spermato- 
phytes  that  this  differentiation  of  tissues  becomes  extreme, 
and  tissues  are  organized  which  differ  decidedly  from 
parenchyma.  This  differentiation  means  division  of  labor, 
and  the  more  highly  organized  the  body  the  more  tissues 
there  are. 

All  the  other  tissues  are  derived  from  parenchyma,  and 
as  the  work  of  nutrition  and  of  reproduction  is  always 
retained  by  the  parenchyma  cells,  the  derived  tissues  are 
for  mechanical  rather 
than  for  vital  purposes. 
There  is  a  long  list  of 
these  derived  and  me- 
chanical tissues,  some  of 
them  being  of  general 
occurrence,  and  others 
more  restricted,  and 
there  is  every  gradation 

between    them    and    the       Fig.  2&4.    Parenchyma  and  sclerenchyma  from 

parenchyma  from  which        the  8tem  of  **""<*• in  cross-section.-CHAM- 

r  J  BERLAIX. 

they    have    come.      \Ve 

shall  note  only  a  few  which  are  distinctly  differentiated 

and  which  are  common  to  all  vascular  plants. 

149.  Parenchyma. — The  parenchyma  of  the  vascular  plants 
is  typically  made  up  of  cells  which  have  thin  walls  and  whose 
three  dimensions  are  approximately  equal  (Figs.  264,  265), 
though  sometimes  they  are  elongated.  Until  abandoned, 
such  cells  contain  very  active  protoplasm,  and  it  is  in  them 
that  nutritive  work  and  cell  division  are  carried  on.  So 
long  as  these  cells  retain  the  power  of  cell  division  the 
tissue  is  called  meristem,  or  it  is  said  to  be  meristematic, 
from  a  Greek  word  meaning  "  to  divide."  When  the  cells 
stop  dividing,  the  tissue  is  said  to  be  permanent.  The 
growing  points  of  organs,  as  stems,  roots,  and  leaves,  are 
composed  of  parenchyma  which  is  meristematic  (Figs.  266, 
274),  and  meristem  occurs  wherever  growth  is  going  on. 


282 


PLANT   STEUCTUKES 


150.  Mestome  and  stereome. — When  the  plant  body  be- 
corned  complex  a  conductive  system  is  necessary,  so  that 
the  different  regions  of  the  body  may  be  put  into  communi- 
cation. The  material  absorbed 
by  the  roots  must  be  carried  to 
the  leaves,  and  the  food  manu- 
factured in  the  leaves  must 
be  carried  to  regions  of  growth 
and  storage.  This  business  of 
transportation  is  provided  for 
by  the  specially  organized  ves- 
sels referred  to  in  preceding 
chapters,  and  all  conducting  tis- 
sue, of  whatever  kind,  is  spoken 
of  collectively  as  mestome. 

If  a  complex  body  is  to  main- 
tain its  form,  and  especially  if 
it  is  to  stand  upright  and  be- 
come large,  it  must  develop 
structures  rigid  enough  to  fur- 
nish mechanical  support.  All 
the  tissues  which  serve  this  pur- 
pose are  collectively  known  as 
stereome. 

The  sporophyte  body  of 
Pteridophytes  and  Spermato- 
phytes,  therefore,  is  mostly 
made  up  of  living  and  working  parenchyma,  which  is 
traversed  by  mechanical  mestome  and  stereome. 

151.  Dicotyl  and  Conifer  stems. — The  stems  of  these  two 
groups  are  so  nearly  alike  in  general  plan  that  they  may 
be  considered  together.  In  fact,  the  resemblances  were 
once  thought  to  be  so  important  that  these  two  groups 
were  put  together  and  kept  distinct  from  Monocotyledons  ; 
but  this  was  before  the  gametophyte  structures  were 
known  to  bear  very  different  testimony. 


Fig.  265.  Same  tissues  as  in  pre- 
ceding figure,  in  longitudinal  sec- 
tion, the  parenchyma  showing 
nuclei. — Chamberlain. 


DIFFEKEMTlATION   OF  TISSUES 


283 


At  the  apex  of  the  growing  stem  there  is  a  group  of 
active  meristem  cells,  from  which  all  the  tissues  are  de- 
rived (Fig.  266).  This  group  is  known  as  the  apical  group. 
Below  the  apical  group  the  tissues  and  regions  of~Ehestem 
begin  to  appear,  and  still  farther  down  they  become  dis- 
tinctly differentiated,  passing  into  permanent  tissue,  the 
apical  group  by  its 
divisions  continually 
adding  to  them  and 
increasing  the  stem 
in  length. 

Just  behind  the 
apical  group,  the 
cells  begin  to  give  the 
appearance  of  being 
organized  into  three 
great  embryonic  re- 
gions, the  cells  still 
remaining  meristem- 
atic  (Fig.  266).  At 
the  surface  there  is  a 
single  layer  of  cells 
distinct    from    those 

within,  known  as  the  dermatogen,  or  "  skin-producer,"  as 
farther  down,  where  it  becomes  permanent  tissue,  it  is  the 
epidermis.  In  the  center  of  the  embryonic  region  there 
is  organized  a  solid  cylinder  of  cells,  distinct  from  those 
around  it,  and  called  the  plerome,  meaning  "  that  which 
fills  up."  Farther  down,  where  the  plerome  passes  into 
permanent  tissue,  it  is  called  the  central  cylinder  or  stele 
("column").  Between  the  plerome  and  dermatogen  is 
a  tissue  region  called  the  periblem,  meaning  "  that  which 
is  put  around,"  and  when  it  becomes  permanent  tissue  it 
is  called  the  cortex,  meaning  "bark"  or  "rind." 

Putting  these  facts  together,  the  general  statement  is 
that  at  the  apex  there  is  the  apical  group  of  meristem  cells  ; 


Fig.  266.  Section  through  growing  point  of  stem  of 
Bippuris  :  below  the  growing  point,  composed 
of  a  uniform  meristem  tissue,  the  three  embry- 
onic regions  are  outlined,  showing  the  dermato- 
gen (d,  d),  the  central  plerome  (p,p),  and  be- 
tween them  the  periblem. — After  De  Bart. 


284 


PLANT   STRUCTURES 


below  them  are  the  three  embryonic  regions,  dermatogen, 
periblem,  and  plerome  ;  and  farther  below  these  three 
regions  pass  into  permanent  tissue,  organizing  the  epider- 
mis, cortex,  and  stele.  The  three  embryonic  regions  are 
usually  not  so  distinct  in  the  Conifer  stem  as  in  the  Dico- 
tyl  stem,  but  both  stems  have  epidermis,  cortex,  and  stele. 
Epidermis. — The  epidermis  is  a  protective  layer,  whose 
cells  do  not  become  so  much  modified  but  that  they  may 
be  regarded  as  parenchyma.  It  gives  rise  also  to  super- 
ficial parts,  as  hairs,  etc.  In  the  case  of  trees,  the  epidermis 
does  not  usually  keep  up  with  the  increasing  diameter,  and 
disappears.  This  puts  the  work  of  protection  upon  the 
cortex,  which  organizes  a  superficial  tissue  called  cork,  a 
prominent  part  of  the  structure  known  as  bark. 

Cortex. — The  cortex  is  characterized  by  containing 
much  active  parenchyma,  or  primitive  tissue,  being  the 
chief  seat  of  the  life  activities  of  the  stem.  Its  superficial 
cells,  at  least,  contain  chlorophyll  and  do  chlorophyll  work, 
while  its  deeper  cells  are  usually  temporary  storage  places 

for  food.  The  cortex  is  also  char- 
acterized by  the  development  of 
stereome,  or  rigid  tissues  for  me- 
chanical support.  The  stereome 
may  brace  the  epidermis,  forming 
the  Jiypodermis  ;  or  it  may  form 
bands  and  strands  within  the  cor- 
tex ;  in  fact,  its  amount  and  ar- 
rangement differ  widely  in  differ- 
ent plants. 

The  two  principal  stereome  tis- 
sues are  collenchyma  and  scleren- 
chyma,  meaning  "  sheath-tissue  " 
and  "  hard-tissue "  respectively. 
In  collenchyma  the  cells  are  thick- 
ened at  the  angles  and  have  very  elastic  walls  (Fig.  267), 
making  the  tissue  well  adapted  for  parts  which  are  growing 


Fig.  267.  Some  collenchyma 
cells  from  the  stem  of  a  com- 
mon dock  (Bitmex),  showing 
the  cells  thickened  at  the 
angles.  —Ch  AMBERLAIN. 


DIFFERENTIATION   OF  TISSUES 


285 


in  length.  The  chief  mechanical  tissue  for  parts  which 
have  stopped  growing  in  length  is  sclerenchyma  (Figs.  264, 
265).  The  cells  are  thick-walled,  and  usually  elongated 
and  with  tapering  ends,  including  the  so-called  "fibers." 


Fig.  268.  Sections  through  an  open  collateral  vascular  bundle  from  a  sunflower  stem; 
A,  cross-section;  B.  longitudinal  section;  the  letters  in  both  referring  to  the  same 
structures;  M,  pith;  X,  xylem,  containing  spiral  (8,  «')  and  pitted  (t,  t')  vessels; 
C,  cambium;  P,  phloem,  containing  sieve  vessels  (sb);  b,  a  mass  of  bast  fibers  or 
sclerenchyma;  ic,  pith  rays  between  the  bundles;  e,  the  bundle  sheath;  R,  cor- 
tex.—After  Vines. 

Stele.—  The  characteristic  feature  of  the  stele  or  central 
cylinder  is  the  development  of  the  mestome  or  vascular 
37 


286 


PLANT  STRUCTURES 


tissues,  of  which  there  are  two  prominent  kinds.  The 
traclieary  vessels  are  for  water  conduction,  and  are  cells 
with  heavy  walls  and  usually  large  diameter  (Fig.  268). 
The  thickening  of  the  walls  is  not  uniform,  giving  them  a 
very  characteristic  appearance,  the  thickening  taking  the 
form  of  spiral  bands,  rings,  or  reticulations  (Fig.  268,  B). 
Often  the  reticulation  has  such  close  meshes  that  the  cell 
wall  has  the  appearance  of  being  covered  with  thin  spots, 
and  such  cells  are  called  "  pitted  vessels."  The  vessels  with 
spirals  and  rings  are  usually  much  smaller  in  diameter  than 
the  pitted  ones.  The  true  traclieary  cells  are  more  or  less 
elongated  and  without  tapering  ends,  fitting  end  to  end 
and  forming  a  continuous  longitudinal  series,  suggesting  a 
trachea,  and  hence  the  name.     In  the  Conifers  there  are 

no  true  traclieary  cells,  as  in 
the  Dicotyledons,  except  a  few 
small  spiral  vessels  which  are 
formed  at  first  in  the  young 
stele,  but  the  tracheary  tissue 
is  made  up  of  tracheitis,  mean- 
ing "trachea -like,"  differing 
from  trachea  or  true  traclieary 
vessels  in  having  tapering  ends 
and  in  not  forming  a  continu- 
ous series  (Fig.  269).  The  walls 
of  these  tracheids  are  "pitted" 
in  a  way  which  is  characteristic 
of  Gymnosperms,  the  "pits" 
appearing  as  two  concentric 
rings,  called  "bordered  pits." 

The  other  prominent  mes- 
tome  tissue  developed  in  the 
stele  is  the  sieve  vessels,  for  the 
conduction  of  organized  food,  chiefly  proteids  (Fig.  268). 
Sieve  cells  are  so  named  because  in  their  walls  special  areas 
are  organized  which  are  perforated  like  the  lid  of  a  pepper- 


Fig.  269.  Tracheids  from  wood  of 
pine,  showing  tapering  ends  and 
bordered  pits.— Chamberlain. 


DIFFERENTIATION  OF  TISSUES  287 

box  or  a  "  sieve."  These  perforated  areas  are  the  sieve- 
plates,  and  through  them  the  vessels  communicate  with 
one  another  and  with  the  adjacent  tissue. 

The  tracheary  and  sieve  vessels  occur  in  separate 
strands,  the  tracheary  strand  being  called  xylem  ("  wood  *), 
the  sieve  strand  phloem  ("bark  ").  A  xylem  and  a  phloem 
strand  are  usually  organized  together  to  form  a  vascular 
bundle,  and  it  is  these  fiber-like  bundles  which  are  found 
traversing  the  stems  of  all  vascular  plants  and  appearing 
conspicuously  as  the  veins  of  leaves.  Among  the  Dicotyls 
and  Conifers  the  vascular  bundles  appear  in  the  stele  in 
such  a  way  as  to  outline  a  hollow  cylinder  (Fig.  216),  the 
xylem  of  each  bundle  being  toward  the  center,  the  phloem 
toward  the  circumference  of  the  stem.  The  undifferenti- 
ated parenchyma  of  the  stele  which  the  vascular  cylinder 
incloses  is  called  the  pith.  In  older  parte  of  the  stem  the 
pith  is  often  abandoned  by  the  activities  of  the  plant,  and 
either  remains  as  a  dead  spongy  tissue,  or  disappears  en- 
tirely, leaving  a  hollow  stem.  Between  the  bundles  form- 
ing the  vascular  cylinder  there  is  also  undifferentiated 
parenchyma,  and  as  it  seems  to  extend  from  the  pith  out 
between  the  bundles  like  "rays  from  the  sun,"  the  rays 
are  called  pith  rays. 

Such  vascular  bundles  as  described  above,  in  which  the 
xylem  and  phloem  strands  are  "  side-by-side  "  upon  the  same 
radius,  are  called  collateral  (Fig.  270).  One  of  the  pecul- 
iarities of  the  collateral  bundles  of  Dicotyls  and  Conifers, 
however,  is  that  when  the  two  strands  of  each  bundle  are 
organized  some  meristem  is  left  between  them.  This  means 
that  between  the  strands  the  work  of  forming  new  cells  can 
go  on.  Such  bundles  are  said  to  be  open ;  and  the  open 
collateral  bundle  is  characteristic  of  the  stems  of  the  Dico- 
tyls and  Conifers. 

The  meristem  between  the  xylem  and  phloem  of  the 
open  bundle  is  called  cambium  (Figs.  268,  270).  The  cam- 
bium also  extends  across  the  pith  rays  between  the  bundles, 


288 


PLANT   STRUCTURES 


connecting  the  cambium  in  the  bundles,  and  thus  forming 
a  cambium  cylinder,  which  separates  the  xylem  and  phloem 
of  the  vascular  cylinder.     This  cambium  continues  the  f  or- 


Fig.  270.  Cross-section  of  open  collateral  vascular  bundle  from  stem  of  castor-oil 
plant  (Bicinus),  showing  pith  cells  (to),  xylem  containing  spiral  (t)  and  pitted  (g) 
vessels,  cambium  of  bundle  (c)  and  of  pith  rays  (cb),  phloem  containing  sieve  vea 
sels  (y),  three  bundles  of  bast  fibers  or  sclerenchyma  (6),  the  bundle  sheath  con- 
taining starch  grains,  and  outside  of  it  parenchyma  of  the  cortex  (?•)• — After  Sachs. 

mation  of  xylem  tissue  on  the  one  side  and  phloem  tissue 
on  the  other  in  the  bundles,  and  new  parenchyma  between 
the  bundles,  and  so  the  stem  increases  in  diameter.  If  the 
stem  lives  from  year  to  year  the  addition  made  by  the  cam- 
bium each  season  is  marked  off  from  that  of  the  previous 
season,  giving  rise  to  the  so-called  growth  rings  or  annual 
rings,  so  conspicuous  a  feature  of  the  cross-section  of  tree 


DIFFERENTIATION   OF  TISSUES 


289 


trunks  (Fig.  217).  This  continuous  addition  to  the  vessels 
increases  the  capacity  of  the  stem  for  conduction,  and  per- 
mits the  further  extension  of  branches  and  a  larger  display 
of  leaves. 

The  annual  additions  to  the  xylem  are  added  to  the  in- 
creasing mass  of  wood.  The  older  portions  of  the  xylem 
mass  are  gradually  abandoned  by  the  ascending  water 
("sap"),  often  change  in  color,  and  form  the  heart-wood. 
The  younger  portion,  through  which  the  sap  is  moving,  is 
the  sap-iuood.  It  is  evident,  however,  that  the  annual  ad- 
ditions to  the  phloem  are  not  in  a  position  for  permanency. 
The  new  phloem  is  deposited  inside  of  the  old,  and  this,  to- 
gether with  the  new  xylem,  presses  upon  the  old  phloem, 
which  becomes  ruptured  in  various  ways,  and  rapidly  or 
very  gradually  peels  off,  being  constantly  renewed  from 
within.  It  is  the  protecting  layers  of  cork  (see  this  section 
under  Cortex),  the  old  phloem,  and  the  new  phloem  down 
to  the  cambium,  which  constitute  the  so-called  bark  of 
trees,  a  structure  exceedingly  complex  and  extremely  vari- 
able in  different  trees. 

The  stele  also  frequently  develops  stereome  tissue  in  the 
form  of  sclerenchyma.  These  thick-walled  fibers  are  often 
closely  associated  with  one  or  both  of  the  vascular  strands 
of  the  bundles  (Fig.  270),  and  lead  to  the  old  name  fibro- 
vascular  bundles. 

To  sum  up,  the  stems  of  Dicotyledons  and  Conifers  are 
characterized  by  the  development  of  a  vascular  cylinder,  in 
which  the  bundles  are  collateral  and  open,  permitting 
increase  in  diameter,  extension  of  the  branch  system,  and 
a  continuous  increase  in  leaf  display. 

152.  Monocotyl  stems. — In  the  stems  of  Monocotyledons 
there  is  the  same  apical  development  and  differentiation 
(Fig.  266).  The  characteristic  difference  from  the  Dicotyl 
and  Conifer  type,  just  described,  is  in  connection  with  the 
development  of  the  vascular  bundles  in  the  stele.  Instead 
of  outlining  a  hollow  cylinder,  the  bundles  are  scattered 


290 


PLANT   STRUCTURES 


through  the  stele  (Fig.  214).  This  lack  of  regularity  would 
interfere  with  the  organization  of  a  cambium  cylinder,  and 
we  find  the  bundles  collateral  but  closed — that  is,  with  no 
meristem  left  between  the  xylem  and  phloem  (Fig.  271). 


Pig.  271.  Cross-section  of  a  closed  collateral  bundle  from  the  stem  of  corn,  showing 
the  xylem  with  annular  (r),  spiral  («),  and  pitted  (g)  vessels;  the  phloem  contain- 
ing sieve  vessels  (v),  and  separated  from  the  xylem  by  no  intervening  cambium; 
both  xylem  and  phloem  surrounded  by  a  mass  of  sclerenchyma  (fibers);  and  in- 
vesting vessels  and  fibers  the  parenchyma  (p)  of  the  pith-like  tissue  through 
which  the  bundles  are  distributed. — After  Sachs. 


This  lack  of  cambium  means  that  stems  living  for  sev- 
eral years  do  not  increase  in  diameter,  but  become  columnar 


DIFFERENTIATION   OF  TISSUES 


291 


shafts,  as  in  the  palm,  rather  than  much  elongated  cones. 
It  also  means  lack  of  ability  to  develop  an  extending  branch 
system  or  to  display  more  numerous  leaves  each  year.  The 
palm  may  be  taken  as  a  typical  result  of  such  a  structure, 
with  its  columnar  and  unbranched  trunk,  and  its  foliage 
crown  containing  about  the  same  number  of  leaves  each  year. 

The  lack  of  regular  arrangement  of  the  bundles  also 
prevents  the  outlining  of  a  pith  region  or  the  organization 
of  definite  pith  rays.  The  failure  to  increase  in  diameter 
also  precludes  the  necessity  of  bark,  with  its  protective  cork 
constantly  renewed,  and  its  sloughing-off  phloem. 

To  sum  up,  the  stems  of  the  Monocotyledons  are 
characterized  by  the  vascular  bundles  not  developing  a 
cylinder  or  any  regular  arrangement,  and  by  collateral  and 
closed  bundles,  which  do  not  permit  increase  in  diameter, 
or  a  branch  system,  or  increase  in  leaf  display. 

153.  Pteridophyte  stems. — The  stems  of  Pteridophytes 
are  quite  different  from  those  of  Spermatophytes.  While 
the  large  Club -mosses  {Lyco- 
podium)  and  Isoetes  usually 
have  an  apical  group  of  meris- 
tem  cells,  as  among  the  Seed- 
plants,  the  smaller  Club-mosses 
(Selaginella),  Ferns,  and  Horse- 
tails usually  have  a  single  api- 
cal cell,  whose  divisions  give 
rise  to  all  the  cells  of  the  stem. 
Generally  also  a  dermatogen  is 
not  organized,  and  in  such 
cases  there  is  no  true  epidermis, 
the  cortex  developing  the  ex- 
ternal protective  tissue.  In  the  cortex  there  is  usually  an 
extensive  development  of  stereome,  in  the  form  of  scleren- 
chyma  (Fig.  272),  the  stele  furnishing  little  or  none,  and 
the  vascular  bundles  not  adding  much  to  the  rigidity,  as 
they  do  in  the  Seed-plants. 


Fig.  272.  Diagram  of  tissues  in  cross- 
section  of  stem  of  a  fern  (Pteris), 
showing  two  masses  of  scleren- 
chyma  (st),  between  and  about 
which  are  vascular  bundles.  — 
Chamberlain. 


292  PLANT  STKUCTUKES 

In  Equisetum  and  Isoetes  the  vascular  bundles  may  be 
said  to  be  collateral,  as  in  the  Seed-plants,  but  the  charac- 
teristic Pteridophyte  type  is  very  different.  In  fact,  the 
vascular  masses  can  hardly  be  compared  with  the  bundles 
of  the  Seed-plants,  although  they  are  called  bundles  for 
convenience.  In  the  stele  one  or  more  of  these  bundles 
are  organized  (Fig.  272),  the  tracheary  vessels  (xylem)  being 
in  the  center  and  completely  invested  by  the  sieve  vessels 


Fig.  273.  Cross-section  of  concentric  vascular  bundle  of  a  fern  (P(eris):  the  single 
row  of  shaded  cells  investing  the  others  is  the  bundle  sheath ;  the  large  and  heavy- 
walled  cells  within  constitute  the  xylem;  and  between  the  xylem  and  the  bundle 
sheath  is  the  phloem. — Chamberlain. 

(phloem).  This  is  called  the  concentric  bundle  (Fig.  273), 
as  distinguished  from  the  collateral  bundles  of  Seed-plants, 
and  is  characteristic  of  Pteridophyte  stems. 


DIFFERENTIATION  OF  TISSUES 


293 


154.  Roots. — True  roots  appear  only  in  connection  with 
the  vascular  plants  (Pteridophytes  and  Spermatophytes)  ; 


Fig.  274.  Section  through  root- 
tip  ofPteris:  the  cell  with 
a  nucleus  is  the  single  apical 
cell,  which  in  front  has  cut 
off  cells  which  organize  the 
root-cap. — Chamberlain. 

and  in  all  of  them  the 
structure  is  essential- 
ly the  same,  and  quite 
different  from  stem 
structure.  A  single 
apical  cell  (in  most 
Pteridophytes)  (Fig. 
274)  or  an  apical 
group  (in  Spermato- 
phytes) usually  gives 
rise  to  the  three  em- 
bryonic regions — der- 
matogen,  periblem, 
and  plerome  (Fig. 
275).  A  fourth  re- 
gion, however,  pecul- 
iar to  root,  is  usually  added.  The  apical  cell  or  group  cuts 
off  a  tissue  in  front  of  itself  (Fig.  274),  known  as  the  calyp- 
trogen,  or  "cap  producer,"  for  it  organizes  the  root-cap, 
which  protects  the  delicate  meristem  of  the  growing  point. 


Fro.  275.  A  longitudinal  section  through  the  root- 
tip  of  spiderwort,  showing  the  plerome  {pi), 
surrounded  by  the  periblem  ( p),  outside  of 
periblem  the  epidermis  (e)  which  disappears  in 
the  older  parts  of  the  root,  and  the  prominent 
root-cap  (c). — Land. 


294  PLANT   STRUCTURES 

Another  striking  feature  is  that  in  the  stele  there  is 
organized  a  single  solid  vascular  cylinder,  forming  a  tough 
central  axis  (Fig.  277),  from  which  the  usually  well-devel- 
oped cortex  can  be  peeled  off  as  a  thick  rind.  In  this  vas- 
cular axis,  which  is  called  "  a  bundle  "  for  convenience  but 
does  not  represent  the  bundle  of  Seed-plant  stems,  the  ar- 
rangement of  the  xylem  and  phloem  is  entirely  unlike  that 


r 

Fig.  276.   Cross-section  of  the  vascular  axis  of  a  root,  showing  radiate  type  of  bundle, 
the  xylem  (p)  and  phloem  (p/n  alternating.  -Alter  Saciib. 

found  in  stems.  The  xylem  is  in  the  center  and  sends  out 
a  few  radiating  arms,  between  which  are  strands  of  phloem, 
forming  the  so-called  radiate  bundle  (Fig.  276).  This 
arrangement  brings  the  tracheary  vessels  (xylem)  to  the 
surface  of  the  bundle  region,  which  is  not  true  of  either 
the  concentric  or  collateral  bundle.  This  seems  to  be  asso- 
ciated with  the  fact  that  the  xylem  is  to  receive  and  conduct 
the  water  absorbed  from  the  soil.  It  should  be  said  that 
this  characteristic  bundle  structure  of  the  root  appears  only 


DIFFERENTIATION   OF  TISSUES 


295 


in  young  and  active  roots.  In  older  ones  certain  secondary 
changes  take  place  which  obscure  the  structure  and  result 
in  a  resemblance  to  the  stem. 

The  origin  of  branches  in 
roots  is  also  peculiar.  In  stems 
branches  originate  at  the  sur- 
face, involving  epidermis,  cor- 
tex, and  vascular  bundles,  such 
an  origin  being  called  exogenous 
("produced  outside");  but  in 
roots  branches  originate  on 
the  vascular  cylinder,  burrow 
through  the  cortex,  and  emerge 
at  the  surface  (Fig.  277).  If  the 
cortex  be  stripped  off  from  a 
root  with  branches,  the  branches 
are  left  attached  to  the  woody 
axis,  and  the  cortex  is  found 
pierced  with  holes  made  by  the  burrowing  branches.  Such 
an  origin  is  called  endogenous,  meaning  "  produced  within." 


me. 


Fig.  277.  Endogenous  origin  of 
root  branches,  showing  them  («) 
arising  from  the  central  axis  (/) 
and  breaking  through  the  cortex 
(?•).— After  Vines. 


Fig  278  A  section  through  the  leaf  of  lily,  (showing  upper  epidermis  {itt  i.  lower  epi- 
dermis (M  with  its  stomata(«0,  mesophy.ll  .dotted  cells)  composed  of  the  palisade 
region  (p)  and  the  spongy  region  («p)  with  air  spaces  among  the  cells,  and  two 
veins  (»)  cut  across.— From  "  Plant  Relations." 


29G  PLANT   STRUCTURES 

To  sum  up  the  peculiarities  of  the  root,  it  may  be  said 
to  develop  a  root-cap,  to  have  a  solid  vascular  cylinder  in 
which  the  xylem  and  phloem  are  arranged  to  form  a  bundle 
of  the  radiate  type,  and  to  branch  endogenously. 

155.  Leaves. — Leaves  usually  develop  from  an  apical 
region  in  the  same  general  way  as  do  stems  and  roots, 
modified  by  their  common  dorsiventral  character.  Com- 
paring the  leaf  of  an  ordinary  seed-plant  with  its  stem,  it 
will  be  noted  that  the  three  regions  are  represented  (Fig. 
278):  (1)  the  epidermis;  (2)  the  cortex,  represented  by 
the  mesophyll ;  (3)  the  stele,  represented  by  the  veins. 

In  the  case  of  collateral  bundles,  where  in  the  stem  the 
xylem  is  always  toward  the  center  and  the  phloem  is  toward 
the  circumference,  in  the  leaves  the  xylem  is  toward  the 
upper  and  the  phloem  toward  the  lower  surface. 


CHAPTER  XVI 

PLANT  PHYSIOLOGY 

156.  Introductory. — Plants  may  be  studied  from  several 
points  of  view,  each  of  which  has  resulted  in  a  distinct 
division  of  Botany.  The  study  of  the  forms  of  plants  and 
their  structure  is  Moephology,  and  it  is  this  phase  of  Bot- 
any which  has  been  chiefly  considered  in  the  previous  chap- 
ters. The  study  of  plants  at  work  is  Physiology,  and  as 
structure  is  simply  preparation  for  work,  the  preceding 
chapters  have  contained  some  Physiology,  chiefly  in  refer- 
ence to  nutrition  and  reproduction.  The  study  of  the  clas- 
sification of  plants  is  Taxonomy,  and  in  the  preceding 
pages  the  larger  groups  have  been  outlined.  The  study  of 
plants  as  to  their  external  relations  is  Ecology,  a  subject 
which  will  be  presented  in  the  following  chapter,  and  which 
is  the  chief  subject  of  Plant  Relations.  The  study  of  the 
diseases  of  plants  and  their  remedies  is  Pathology  ;  their 
study  in  relation  to  the  interests  of  man  is  Economic 
Botany. 

Besides  these  general  subjects,  which  apply  to  all  plants, 
the  different  groups  form  the  subjects  of  special  study.  The 
study  of  the  Morphology,  Physiology,  or  Taxonomy  of  the 
Bacteria  is  Bacteriology;  of  the  Algge,  Algology ;  of  the 
Fungi,  Mycology;  of  the  Bryophytes,  Bryology;  of  the 
fossil  plants,  Paleobotany  or  PaheopJtytology,  etc. 

In  the  present  chapter  it  is  the  purpose  to  give  a  very 
brief  outline  of  the  great  subject  of  Plant  Physiology,  not 
with  the  expectation  of  presenting  its  facts  adequately,  but 
with  the  hope  that  the  important  field  thus  presented  may 

297 


298  PLANT   STRUCTURES 

attract  to  further  study.    It  is  merely  the  opening  of  a  door 
to  catch  a  fleeting  glimpse. 

A  common  division  of  the  subject  presents  it  under  five 
heads  :  (1)  Stability  of  form,  (2)  Nutrition,  (3)  Inspira- 
tion, (4)  Movement,  (5)  Reproduction. 

STABILITY    OF    FORM 

157.  Turgidity. — It  is  a  remarkable  fact  that  plants  and 
parts  of  plants  composed  entirely  of  cells  Avith  very  thin  and 
delicate  walls  are  rigid  enough  to  maintain  their  form. 
It  has  already  been  noted  (see  §  20)  that  such  active  cells 
exert  an  internal  pressure  upon  their  walls.  This  seems  to 
be  due  to  the  active  absorption  of  liquid,  which  causes  the 
very  elastic  walls  to  stretch,  as  in  the  "blowing  up  "  of  a 
bladder.  In  this  way  each  gorged  and  distended  cell  be- 
comes comparatively  rigid,  and  the  mass  of  cells  retains  its 
form.  It  seems  evident  that  the  active  protoplasm  greedily 
pulls  liquid  through  the  wall  and  does  not  let  it  escape  so 
easily.  If  for  any  reason  the  protoplasm  of  a  gorged  cell 
loses  its  hold  upon  the  contained  liquid  the  cell  collapses. 

158.  Tension  of  tissues. — The  rigidity  which  comes  to 
active  parenchyma  cells  through  their  turgidity  is  increased 
by  the  tensions  developed  by  adjaceDt  tissues.  For  exam- 
ple, the  internal  and  external  tissues  of  a  stem  are  apt  to 
increase  in  volume  at  different  rates ;  the  faster  will  pull 
upon  the  slower,  and  the  slower  will  resist,  and  thus  be- 
tween the  two  a  tension  is  developed  which  helps  to  keep 
them  rigid.  This  is  strikingly  shown  by  splitting  a  dande- 
lion stem,  when  the  inner  tissue,  relieved  somewhat  from 
the  resistance  of  the  outer,  elongates  and  causes  the  strip 
to  become  strongly  curved  outward  or  even  coiled.  Experi- 
ments with  strips  from  active  twigs,  including  the  pith, 
will  usually  demonstrate  the  same  curve  outward.  Tension 
of  tissues  is  chiefly  developed,  of  course,  where  elongation 
is  taking  place. 


PLANT   PHYSIOLOGY  299 

159.  Stereome. — When  growth  is  completed,  cell  walls 
lose  their  elasticity,  turgidity  becomes  less,  and  therefore 
tensions  diminish,  and  rigidity  is  supplied  by  special  ster- 
eome tissues,  chief  among  which  is  sclerenchyma.  An- 
other stereome  tissue  is  collenchyma,  which  on  account  of 
its  elastic  walls  can  be  used  to  supplement  turgidity  and 
tension  where  elongation  is  still  going  on.  For  a  fuller 
account  of  stereome  tissues  see  8  150. 


NUTRITION 

160.  Food. — Plant  food  must  contain  carbon  (C),  hydro- 
gen (H),  oxygen  (0),  and  nitrogen  (N),  and  also  more 
or  less  of  other  elements,  notably  sulphur,  phosphorus, 
potassium,  calcium,  magnesium,  and  iron.  In  the  case 
of  green  plants  these  elements  are  obtained  from  inor- 
ganic compounds  and  food  is  manufactured  ;  while  plants 
without  chlorophyll  obtain  their  food  already  organized. 
The  sources  of  these  elements  for  green  plants  are  as 
follows:  Carbon  from  carbon  dioxide  (C02)  of  the  air; 
hydrogen  and  oxygen  from  water  (H30) ;  and  nitrogen 
and  the  other  elements  from  their  various  salts  which 
occur  in  the  soil  and  are  dissolved  in  the  water  which 
enters  the  plant. 

All  of  these  substances  must  present  themselves  to 
plants  in  the  form  of  a  gas  or  a  liquid,  as  they  must  pass 
through  cell  walls  ;  and  the  processes  of  absorption  have 
to  do  with  the  taking  in  of  the  gas  carbon  dioxide  and  of 
water  in  which  the  necessary  salts  are  dissolved. 

161.  Absorption. — Green  plants  alone  will  be  considered, 
as  the  unusual  methods  of  securing  food  have  been  men- 
tioned in  Chapter  VII.  For  convenience  also,  only  terres- 
trial green  plants  will  be  referred  to,  as  it  is  simple  to 
modify  the  processes  to  the  aquatic  habit,  where  the  sur- 
rounding water  supplies  what  is  obtained  by  land  plants 
from  both  air  and  soil. 


300  PLANT  STRUCTURES 

In  such  plants  the  carbon  dioxide  is  absorbed  directly 
from  the  air  by  the  foliage  leaves,  whose  expanse  of  surface 
is  as  important  for  this  purpose  as  for  exposing  chlorophyll 
to  light.  When  the  work  of  foliage  leaves  is  mentioned  it 
must  always  be  understood  that  it  applies  as  well  to  any 
green  tissue  displayed  by  the  plant. 

The  water,  with  its  dissolved  salts,  is  absorbed  from  the 
soil  by  the  roots.  Only  the  youngest  parts  of  the  root- 
system  can  absorb,  and  the  absorbing  capacity  of  these 
parts  is  usually  vastly  increased  by  the  development  of 
numerous  root  hairs  just  behind  the  growing  tip  (Fig.  194). 
These  root  hairs  are  ephemeral,  new  ones  being  continu- 
ally put  out  as  the  tip  advances,  and  the  older  ones  disap- 
pearing. They  come  in  very  close  contact  with  the  soil 
particles,  and  "suck  in"  the  water  which  invests  each 
particle  as  a  film. 

162.  Transfer  of  water. — The  water  and  its  dissolved  salts 
absorbed  by  the  root-system  must  be  transferred  to  the  foli- 
age leaves,  where  they  are  to  be  used,  along  with  the  carbon 
dioxide,  in  the  manufacture  of  food. 

Having  entered  the  epidermis  of  the  absorbing  rootlets 
the  water  passes  on  to  the  cortex,  and  traversing  it  enters 
the  xylem  system  of  the  central  axis.  In  some  way  this 
transfer  is  accompanied  by  pressure,  known  as  root  pres- 
sure, which  becomes  very  evident  when  an  active  stem  is 
cut  off  near  the  ground.  The  stump  is  said  to  "bleed," 
and  sends  out  water  ("sap")  as  if  there  were  a  force 
pump  in  the  root-system.  This  root  pressure  doubtless 
helps  to  lift  the  water  through  the  xylem  of  the  root  into 
the  stem,  and  in  low  plants  may  possibly  be  able  to  send  it 
to  the  leaves,  but  for  most  plants  this  is  not  possible. 

When  the  water  enters  the  xylem  of  the  root  it  is  in  a 
continuous  system  of  vessels  which  extends  through  the 
stem  and  out  into  the  leaves.  The  movement  of  the  ab- 
sorbed water  through  the  xylem  is  called  the  transpiration 
current,  or  very  commonly  the  "ascent  of  sap."    An  ex- 


PLANT   PHYSIOLOGY  3Q| 

periment  demonstrating  this  ascent  of  sap  and  its  route 
through  the  xylem  will  be  found  described  in  Plant  Rela- 
tions, p.  151.  How  it  is  that  the  transpiration  current 
moves  through  the  xylem  is  not  certainly  known. 

163.  Transpiration. — When  the  water  carrying  dissolved 
salts  reaches  the  mesophyll  cells,  some  of  the  water  and  all 
of  the  salts  are  retained  for  food  manufacture.  However, 
much  more  water  enters  the  leaves  than  is  needed  for  food, 
this  excess  having  been  used  for  carrying  soil  salts.  When 
the  soil  salts  have  reached  their  destination  the  excess  of 
water  is  evaporated  from  the  leaf  surface,  the  process  being 
called  transpiration.  For  an  experiment  demonstrating 
transpiration  see  Plant  Relations,  §  26. 

This  transpiration  is  regulated  according  to  the  needs 
of  the  plant.  If  the  water  is  abundant,  transpiration  is 
encouraged ;  if  the  water  supply  is  low,  transpiration  is 
checked.  One  of  the  chief  ways  of  regulating  is  by  means 
of  the  very  small  but  exceedingly  numerous  stomata  (see  § 
79  [4]),  Avhose  guard  cells  become  turgid  or  collapse  and  so 
determine  the  size  of  the  opening  between  them.  It  has 
been  estimated  that  a  leaf  of  an  ordinary  sunflower  contains 
about  thirteen  million  stomata,  but  the  number  varies  widely 
in  different  plants.  In  ordinary  dorsiventral  leaves  the  sto- 
mata are  much  more  abundant  upon  the  lower  surface  than 
upon  the  upper,  from  which  they  may  be  lacking  entirely. 
In  erect  leaves  they  are  distributed  equally  upon  both  sur- 
faces ;  in  floating  leaves  they  occur  only  upon  the  upper 
surface  ;  in  submerged  leaves  they  are  lacking  entirely. 

The  amount  of  water  thus  evaporated  from  active 
leaves  is  very  great.  It  is  estimated  that  the  leaves  of  a 
sunflower  as  high  as  a  man  evaporate  about  one  quart  of 
water  in  a  warm  day ;  and  that  an  average  oak  tree  in  its 
five  active  months  evaporates  about  twenty-eight  thousand 
gallons.  If  these  figures  be  applied  to  a  meadow  or  a 
forest  the  result  may  indicate  the  large  importance  of  this 

process. 

38 


302  PLANT  STRUCTURES 

164.  Photosynthesis. — This  is  the  process  by  which  car- 
bon dioxide  and  water  are  "broken  up,"  their  elements 
recombined  to  form  a  carbohydrate,  and  some  oxygen  given 
off  as  a  waste  product,  the  mechanism  being  the  chloroplasts 
and  light.  It  has  been  sufficiently  described  in  §  55,  and 
also  in  Plant  Relations,  pp.  28  and  150. 

165.  Formation  of  proteids. — The  carbohydrates  formed 
by  photosynthesis,  such  as  starch,  sugar,  etc.,  contain  car- 
bon, hydrogen,  and  oxygen.  Out  of  them  the  living  cells 
must  organize  proteids,  and  in  the  reconstruction  nitrogen 
and  sulphur,  and  sometimes  phosphorus,  are  added.  This 
work  goes  on  both  in  green  cells  and  other  living  cells,  as 
it  does  not  seem  to  be  entirely  dependent  upon  chloroplasts 
and  light. 

166.  Transfer  of  carbohydrates  and  proteids. — These  two 
forms  of  food  having  been  manufactured,  they  must  be 
carried  to  the  regions  of  growth  or  storage.  In  order  to  be 
transported  they  must  be  in  soluble  form,  and  if  not  already 
soluble  they  must  be  digested,  insoluble  starch  being  con- 
verted into  soluble  sugar,  etc.  In  these  digested  forms 
they  are  transported  to  regions  where  work  is  going  on, 
and  there  they  are  assimilated — that  is,  transformed  into 
the  enormously  complex  working  substance  protoplasm ; 
or  they  are  transported  to  regions  of  storage  and  there  they 
are  reconverted  into  insoluble  storage  forms,  as  starch,  etc. 

These  foods  pass  through  both  the  cortex  and  phloem 
in  every  direction,  but  the  long-distance  transfer  of  pro- 
teids, as  from  leaves  to  roots,  seems  to  be  mainly  through 
the  sieve  vessels. 

RESPIRATION" 

167.  Respiration. — This  is  an  essential  process  in  plants 
as  well  as  in  animals,  and  is  really  the  phenomenon  of 
"breathing."  The  external  indication  of  the  process  is 
the  absorption  of  oxygen  and  the  giving  out  of  carbon  di- 
oxide ;  and  it  goes  on  in  all  organs,  day  and  night.     When 


PLANT   PHYSIOLOGY  393 

it  ceases  death  ensues  sooner  or  later.  By  this  process 
energy,  stored  up  by  the  processes  of  nutrition,  is  liberated, 
and  with  this  liberated  energy  the  plant  works.  It  may  be 
said  that  oxygen  seems  to  have  the  power  of  arousing  pro- 
toplasm to  activity. 

It  is  not  sufficient  for  the  air  containing  oxygen  to  come 
in  contact  merely  with  the  outer  surface  of  a  complex  plant, 
as  its  absorption  and  transfer  would  be  too  slow.  There 
must  be  an  "internal  atmosphere"  in  contact  with  the 
living  cells.  This  is  provided  for  by  the  intercellular 
spaces,  which  form  a  labyrinthine  system  of  passageways, 
opening  at  the  surface  through  stomata  and  lenticels  (pores 
through  bark).  In  this  internal  atmosphere  the  exchange 
of  oxygen  and  carbon  dioxide  is  effected,  the  oxygen  being 
renewed  by  diffusion  from  the  outside,  and  the  carbon 
dioxide  finally  escaping  by  diffusion  to  the  outside. 

MOVEMENT 

168.  Introductory. — In  addition  to  movements  of  mate- 
rial, as  described  above,  plants  execute  movements  depend- 
ent upon  the  activity  of  protoplasm,  which  result  in  change 
of  position.  Naked  masses  of  protoplasm,  as  the  Plas- 
modium of  slime-moulds  (see  §  51),  advance  with  a  sliding, 
snail-like  movement  upon  surfaces  ;  zoospores  and  ciliated 
sperms  swim  freely  about  by  means  of  motile  cilia ;  while 
many  low  plants,  as  Bacteria  (§  52),  Diatoms  (§  34),  Oscil- 
laria  (§  20),  etc.,  have  the  power  of  locomotion. 

When  the  protoplasm  is  confined  within  rigid  walls  and 
tissues,  as  in  most  plants,  the  power  of  locomotion  usually 
disappears,  and  the  plants  are  fixed  ;  but  within  active  cells 
the  protoplasm  continues  to  move,  streaming  back  and 
forth  and  about  within  the  confines  of  the  cell. 

In  the  case  of  complex  plants,  however,  another  kind 
of  movement  is  apparent,  by  which  parts  are  moved  and 
variously  directed,  sometimes  slowly,  sometimes  with  great 


304  PLANT  STRUCTURES 

rapidity.  In  these  cases  the  part  concerned  develops  a 
curvature,  and  by  various  curvatures  it  attains  its  ultimate 
position.  These  curvatures  are  not  necessarily  permanent, 
for  a  perfectly  straight  stem  results  from  a  series  of  cur- 
vatures near  its  apex.  Curvatures  may  be  developed  by 
unequal  growth  on  the  two  sides  of  an  organ,  or  by  unequal 
turgidity  of  the  cells  of  the  two  sides,  or  by  the  unequal 
power  of  the  cell  walls  to  absorb  water. 

169.  Hygroscopic  movements. — These  movements  are  only 
exhibited  by  dry  tissues,  and  hence  are  not  the  direct  result 
of  the  activity  of  protoplasm.  The  dry  walls  absorb  mois- 
ture and  swell  up,  and  if  this  absorption  of  moisture  and 
its  evaporation  is  unequal  on  two  sides  of  an  organ  a  curva- 
ture will  result.  In  this  way  many  seed  vessels  are  rup- 
tured, the  sporangia  of  ferns  are  opened,  the  operculum  of 
mosses  is  lifted  off  by  the  peristome,  the  hair-like  pappus 
of  certain  Composites  is  spread  or  collapsed,  certain  seeds 
are  dispersed  and  buried,  etc.  One  of  the  peculiarities  of 
this  hygroscopic  power  of  certain  cells  is  that  the  result 
may  be  obtained  through  the  absorption  of  the  moisture  of 
the  air,  and  the  hygroscopic  awns  of  certain  fruits  have 
been  used  in  the  manufacture  of  rough  hygrometers 
("  measures  of  moisture  "). 

170.  Growth  movements. — Growth  itself  is  a  great  physi- 
ological subject,  but  certain  movements  which  accompany 
it  are  referred  to  here.  Two  kinds  of  growth  movements 
are  apparent. 

One  may  be  called  nutation,  by  which  is  meant  that  the 
growing  tip  of  an  organ  does  not  advance  in  a  straight 
line,  but  bends  now  toward  one  side,  now  toward  the  other. 
In  this  way  the  tip  describes  a  curve,  which  may  be  a 
circle,  or  an  ellipse  of  varying  breadth  ;  but  as  the  tip  is 
advancing  all  the  time,  the  real  curve  described  is  a  spiral 
with  circular  or  elliptical  cross-section.  The  sweep  of  a 
young  hop-vine  in  search  of  support,  or  of  various  tendrils, 
may  be  taken  as  extreme  illustrations,  but  in  most  cases 


PLANT   PHYSIOLOGY  3Q5 

the  nutation  of  growing  tips  only  becomes  apparent  through 
prolonged  experiment. 

The  other  prominent  growth  movement  is  that  which 
places  organs  in  proper  relations  for  their  work,  sending 
roots  into  the  soil  and  stems  into  the  air,  and  directing 
leaf  planes  in  various  ways.  For  example,  in  the  germina- 
tion of  an  ordinary  seed,  in  whatever  direction  the  parts 
emerge  the  root  curves  toward  the  soil,  the  stem  turns 
upward,  and  the  cotyledons  spread  out  horizontally. 

The  movement  of  nutation  seems  to  be  due  largely  to 
internal  causes,  while  the  movements  which  direct  organs 
are  due  largely  to  external  causes  known  as  stimuli.  Some 
of  the  prominent  responses  to  stimuli  concerned  in  direct- 
ing organs  are  as  follows : 

Heliotropism. — In  this  case  the  stimulus  is  light,  and 
under  its  influence  aerial  parts  are  largely  directed.  Plants 
growing  in  a  window  furnish  plain  illustration  of  helio- 
tropism. In  general  the  stems  and  petioles  curve  toward 
the  light,  showing  positive  heliotropism  (Fig.  279) ;  the 
leaf  blades  are  directed  at  right  angles  to  the  rays  of  light, 
showing  transverse  heliotropism  ;  while  if  there  are  hold- 
fasts or  aerial  roots  they  are  directed  away  from  the  light, 
showing  negative  heliotropism.  The  thallus  bodies  of  ferns, 
liverworts,  etc.,  are  transversely  heliotropic,  as  ordinary 
leaves,  a  position  best  related  to  chlorophyll  work.  If  the 
light  is  too  intense,  leaves  may  assume  an  edgewise  or  pro- 
file position,  a  condition  well  illustrated  by  the  so-called 
"compass  plants."     (See  Plant  Relations,  p.  10.) 

Geotropism. — In  this  case  the  stimulus  is  gravity,  and 
its  influence  in  directing  the  parts  of  plants  is  very  great. 
All  upward  growing  plants,  as  ordinary  stems,  some  leaves, 
etc.,  are  negatively  geotropic,  growing  away  from  the  center 
of  gravity.  Tap-roots  are  notable  illustrations  of  positive 
geotropism,  growing  toward  the  source  of  gravity  with  con- 
siderable force.  Lateral  branches  from  a  main  or  tap-root, 
however,  are  usually  transversely  geotropic. 


Fig.  279.  Sunflower  stems  with  the  upper  part  of  the  stem  sharply  bent  toward  the 
light,  giving  the  leaves  better  exposure,  the  stem  showing  positive  heliotropism. — 
After  Schaffner. 


PLANT  PHYSIOLOGY 


307 


That  these  influences  in  directing  are  very  real  is  testi- 
fied to  by  the  fact  that  when  the  organs  are  turned  aside 
from  their  proper  direction  they  will  curve  toward  it  and 
overcome  a  good  deal  of  resistance  to  regain  it.  Although 
these  curvatures  are  mainly  developed  in  growing  parts, 
even  mature  parts  which  have  been  displaced  may  be 
brought  back  into  position.  For  example,  when  the  stems 
of  certain  plants,  notably  the  grasses,  have  been  prostrated 
by  wind,  etc.,  they  often  can  resume  the  erect  position  under 
the  influence  of  negative  geotropism,  a  very  strong  and  even 
angular  curvature  being  developed  at  certain  joints. 

Hydrotropism. — The  influence  of  moisture  is  very  strong 
in  directing  certain  organs,  notably  absorbing  systems. 
Roots  often  wander  widely  and  in  every  direction  under 
the  guidance  of  hydrotropism,  even  against  the  geotropic 
influence.  Ordinarily  geotropism  and  hydrotropism  act  in 
the  same  direction,  but  it  is  interesting  to  dissociate  them 
so  that  they  may  "pull"  against  one  another.  For  such 
an  experiment  see  Plant  Relations,  p.  91. 

Other  stimuli. — Other  outside  stimuli  which  have  a 
directive  influence  upon  organs  are  chemical  substances 
{cliemotropism),  such  as  direct  sperms  to  the  proper  female 
organ  ;  heat  (thermotropism) ;  water  currents  (rheotropism) ; 
mechanical  contact,  etc.  The  most  noteworthy  illus- 
trations of  the  effect  of  contact  are  furnished  by  tendril- 
climbers.  When  a  nutating  tendril  comes  in  contact  with 
a  support  a  sharp  curvature  is  developed  which  grasps  it. 
In  many  cases  the  irritable  response  goes  further,  the  ten- 
dril between  the  plant  axis  and  the  support  developing  a 
spiral  coil. 

171.  Irritable  movements. — The  great  majority  of  plants 
can  execute  movements  only  in  connection  with  growth,  as 
described  in  the  preceding  section,  and  when  mature  their 
parts  are  fixed  and  incapable  of  further  adjustment.  Cer- 
tain plants,  however,  have  developed  the  power  of  moving 
mature  parts,  the  motile  part  always  being  a  leaf,  such  as 


308 


PLANT  STRUCTURES 


foliage  leaf,  stamen,  etc.  It  is  interesting  to  note  that  these 
movements  have  been  cultivated  by  but  few  families,  nota- 
ble among  them  being  the  Legumes  (§  141). 

These  movements  of  mature  organs,  some  of  which  are 
very  rapid,  are  due  to  changes  in  the  turgidity  of  cells.  As 
already  mentioned  (§  157),  turgid  cells  are  inflated  and 
rigid,  and  when  turgidity  ceases  the  cells  collapse  and  the 
tissue  becomes  flaccid.  A  special  organ  for  varying  tur- 
gidity, known  as  the  pulvi?ius,  is  usually  associated  with 
the  motile  leaves  and  leaflets.  The  pulvinus  is  practically 
a  mass  of  parenchyma  cells,  whose  turgidity  is  made  to  vary 
by  various  causes,  and  leaf -movement  is  the  result. 

The  causes  which  induce  some  movements  are  unknown, 
as  in  the  case  of  Desmodium  gyrans  (see  Plant  Relations, 
p.  49),  whose  small  lateral  leaflets  uninterruptedly  de- 
scribe circles,  completing  a  cycle  in  one  to  three  minutes. 

In  other  cases  the  inciting  cause  is  the  change  from  light 
to  dark,  the  leaves  assuming  at  night  a  very  dif- 
ferent position  from  that  during  the  day.     Dur- 
ing the  day  the  leaflets  are  spread  out  freely, 


Fig.  280.  A  leaf  of  a  sensitive  plant  in  two  conditions:  in  the  figure  to  the  left  the  leaf 
is  fully  expanded,  with  its  four  main  divisions  and  numerous  leaflets  well  spread; 
in  the  figure  to  the  right  is  shown  the  same  leaf  after  it  has  been  "  shocked  "  by 
a  sudden  touch,  or  by  sudden  heat,  or  in  some  other  way;  the  leaflets  have  been 
thrown  together  forward  and  upward,  the  four  main  divisions  have  been  moved 
together,  and  the  main  leaf-stalk  has  been  directed  sharply  downward.— After 

DUCH  AUTRE. 


PLANT  PHYSIOLOGY  309 

while  at  night  they  droop  and  usually  fold  together  (see 
Plant  Relations,  pp.  9,  10).  These  are  the  so-called  nycti- 
tropic  movements  or  "  night  movements,"  which  maybe  ob- 
served in  many  of  the  Legumes,  as  clover,  locust,  bean,  etc. 
In  still  other  cases,  mechanical  irritation  induces  move- 
ment, as  sudden  contact,  heat,  injury,  etc.  Some  of  the 
"  carnivorous  plants  "  are  notable  illustrations  of  this,  es- 
pecially Dionma,  which  snaps  its  leaves  shut  like  a  steel 
trap  when  touched  (see  Plant  Relations,  p.  161).  Among 
the  most  irritable  of  plants  are  the  so-called  "sensitive 
plants,"  species  of  Mimosa,  Acacia,  etc.,  all  of  them  Le- 
gumes. The  most  commonly  cultivated  sensitive  plant  is 
Mimosa  pudica  (Fig.  280),  whose  sensitiveness  to  contact 
and  rapidity  of  response  are  remarkable  (see  Plant  Rela- 
tions, p.  48). 

KEPEODUCTION" 

172.  Reproduction. — The  important  function  of  repro 
duction  has  been  considered  in  connection  with  the  various 
plant  groups.  Among  the  lowest  plants  the  only  method 
of  reproduction  is  cell  division,  which  in  the  complex 
forms  results  in  growth.  In  the  more  complex  plants  va- 
rious outgrowths  or  portions  of  the  body,  as  gemmae,  buds, 
bulbs,  tubers,  various  branch  modifications,  etc.,  furnish 
means  of  propagation.  All  of  these  methods  are  included 
under  the  head  of  vegetative  multiplication,  as  the  plants 
are  propagated  by  ordinary  vegetative  tissues. 

When  a  special  cell  is  organized  for  reproduction,  dis- 
tinct from  the  vegetative  cells,  it  is  called  a  spore,  and  re- 
production by  spores  is  introduced.  The  first  spores  devel- 
oped seem  to  have  been  those  produced  by  the  division  of 
the  contents  of  a  mother  cell,  and  are  called  asexual  spores. 
These  spores  are  scattered  in  various  ways — by  swimming 
(zoospores),  by  floating,  by  the  wind,  by  insects. 

Another  type  of  spore  is  the  sexual  spore,  formed  by 
the  union  of  two  sexual  cells  called  gametes.     The  gametes 


310  PLANT  STKUCTUKES 

seem  to  have  been  derived  from  asexual  spores.  At  first 
the  pairing  gametes  are  alike,  but  later  they  become  differ- 
entiated into  sperms  or  male  cells,  and  eggs  or  female  cells. 

With  the  establishment  of  alternation  of  generations, 
the  asexual  spores  are  restricted  to  the  sporophyte,  and  the 
gametes  to  the  gametophyte.  With  the  further  introduction 
of  heterospory,  the  male  and  the  female  gametes  are  sepa- 
rated upon  different  gametophytes,  which  become  much 
reduced. 

With  the  reduction  of  the  functioning  megaspores  to 
one  in  a  sporangium  (ovule),  and  its  retention,  the  seed  is 
organized,  and  the  elaborate  scheme  of  insect-pollination 
is  developed. 


CHAPTER  XVII 

PLANT     ECOLOGY 

173.  Introductory.— Ecology  has  to  do  with  the  external 
relations  of  plants,  and  forms  the  principal  subject  of  the 
volume  entitled  Plant  Relations,  which  should  be  consulted 
for  fuller  descriptions  and  illustrations.  It  treats  of  the 
adjustment  of  plants  and  their  organs  to  their  physical 
surroundings,  and  also  their  relations  with  one  another 
and  with  animals,  and  has  sometimes  been  called  "plant 
sociology." 

LIFE    RELATIONS 

174.  Foliage  leaves. — The  life  relation  essential  to  foliage 
leaves  is  the  relation  to  light.  This  is  shown  by  their 
positions  and  forms,  as  well  as  by  their  behavior  when 
deprived  of  light.  This  light  relation  suggests  the  answer 
to  very  many  questions  concerning  leaves.  It  is  not  very 
important  to  know  the  names  of  different  forms  and  differ- 
ent arrangements  of  leaves,  but  it  is  important  to  observe 
that  these  forms  and  arrangements  are  in  response  to  the 
light  relation. 

In  general  a  leaf  adjusts  its  own  position  and  its  relation 
to  its  fellows  so  as  to  receive  the  greatest  amount  of  light. 
Upon  erect  stems  the  leaves  occur  in  vertical  rows  which 
are  uniformly  spaced  about  the  circumference.  If  these 
rows  are  numerous  the  leaves  are  narrow  ;  if  they  are  few 
the  leaves  are  usually  broad.  If  broad  leaves  were  associ- 
ated with  numerous  rows  there  would  be  excessive  shading  ; 

311 


312  PLANT  STRUCTURES 

if  narrow  leaves  were  associated  with  few  rows  there  would 
be  waste  of  space. 

It  is  very  common  to  observe  the  lower  leaves  of  a  stem 
long-petioled,  those  above  short-petioled,  and  so  on  until 
the  uppermost  have  sessile  blades,  thus  thrusting  the  blades 
of  lower  leaves  beyond  the  shadow  of  the  upper  leaves. 
There  may  also  be  a  gradual  change  in  the  sree  and  direc- 
tion of  the  leaves,  the  lower  ones  being  relatively  large  and 
horizontal,  and  the  upper  ones  gradually  smaller  and  more 
directed  upward.  In  the  case  of  branched  (compound) 
leaves  the  reduction  in  the  size  of  the  upper  leaves  is  not 
so  necessary,  as  the  light  strikes  between  the  upper  leaflets 
and  reaches  those  below. 

On  stems  exposed  to  light  only  or  chiefly  on  one  side, 
the  leaf  blades  are  thrown  to  the  lighted  side  in  a  variety 
of  ways.  In  ivies,  many  prostrate  stems,  horizontal  branches 
of  trees,  etc.,  the  leaves  brought  to  the  lighted  side  are 
observed  to  form  regular  mosaics,  each  leaf  interfering 
with  its  neighbor  as  little  as  possible. 

There  is  often  need  of  protection  against  too  intense 
light,  against  chill,  against  rain,  etc.,  which  is  provided 
for  in  a  great  variety  of  ways.  Coverings  of  hairs  or  scales, 
the  profile  position,  the  temporary  shifting  of  position, 
rolling  up  or  folding,  reduction  in  size,  etc.,  are  some  of 
the  common  methods  of  protection. 

175.  Shoots. — The  stem  is  an  organ  which  is  mostly 
related  to  the  leaves  it  bears,  the  stem  with  its  leaves  being 
the  shoot.  In  the  foliage-bearing  stems  the  leaves  must  be 
displayed  to  the  light  and  air.  Such  stems  may  be  sub- 
terranean, prostrate,  floating,  climbing,  or  erect,  and  all  of 
these  positions  have  their  advantages  and  disadvantages, 
the  erect  type  being  the  most  favorable  for  foliage  display. 

In  stems  which  bear  scale  leaves  no  light  relation  is 
necessary,  so  that  such  shoots  may  be  and  often  are  sub- 
terranean, and  the  leaves  may  overlap,  as  in  scaly  buds 
and  bulbs.     The  subterranean  position  is  very  favorable 


PLANT  EC<  >!/>(;  Y 


313 


for  food  storage,  and  such  shoots  often  become  modified  as 
food  depositories,  as  in  bulbs,  tubers,  rootstocks,  etc.  In 
the  scaly  buds  the  structure  is  used  for  protection  rather 
than  storage. 

The  stem  bearing  floral  leaves  is  the  shoot  ordinarily 
called  "the  flower,"  whose  structure  and  work  have  been 
sufficiently  described.  Its  adjustments  have  in  view  polli- 
nation and  seed  dispersal,  two  very  great  ecological  sub- 
jects full  of  interesting  details. 

176.  Roots. — Eoots  are  absorbent  organs  or  holdfasts  or 
both,  and  they  enter  into  a  variety  of  relations.  Most 
common  is  the  soil  relation,  and  the  energetic  way  in 
which  such  roots  penetrate  the  soil,  and  search  in  every 
direction  for  water  and  absorb  it,  proves  them  to  be  highly 
organized  members.  Then  there  are  roots  related  to  free 
water,  and  others  to  air,  each  with  its  appropriate  struc- 
ture. More  mechanical  are  the  clinging  roots  (ivies,  etc.), 
and  prop  roots  (screw  pines,  banyans,  etc.),  but  their  adap- 
tation to  the  peculiar  service  they  render  is  none  the  less 
interesting. 

The  above  statements  concerning  leaves,  shoots,  and 
roots  should  be  applied  with  necessary  modifications  to  the 
lower  plants  which  do  not  produce  such  organs.  The 
light  relation  and  its  demands  are  no  less  real  among  the 
Algae  than  among  Spermatophytes,  as  well  as  relations  to 
air,  soil,  water,  mechanical  support,  etc. 

PLANT  ASSOCIATION'S 

177.  Introductory. — Plants  are  not  scattered  at  hap- 
hazard over  the  surface  of  the  earth,  but  are  organized  into 
definite  communities.  These  communities  are  determined 
by  the  conditions  of  living — conditions  which  admit  some 
plants  and  forbid  others.  Such  an  assemblage  of  plants 
living  together  in  similar  conditions  is  a  plant  associatio?i. 
Closely  related  plants  are  the  most  intense  rivals,  as  they 


314  PLANT  STRUCTURES 

make  almost  identical  demands  upon  their  surroundings. 
Hence  it  is  usual  for  a  plant  association  to  be  made  up  of  a 
large  number  of  unrelated  plants. 

There  are  numerous  factors  which  combine  to  deter- 
mine associations,  and  it  is  known  as  yet  only  in  a  vague  way 
how  they  operate. 

178.  Ecological  factors. —  Water. — This  is  a  very  impor- 
tant factor  in  the  organization  of  associations,  which  are 
usually  local  assemblages.  Taking  plants  altogether,  the 
amount  of  water  to  which  they  are  exposed  varies  from 
complete  submergence  to  perpetual  drought,  but  within 
this  range  plants  vary  widely  as  to  the  amount  of  watei 
necessary  for  living. 

Heat. — In  considering  the  general  distribution  of  plants 
over  the  surface  of  the  earth,  great  zones  of  plants  are  out- 
lined by  zones  of  temperature ;  but  in  the  organization  of 
local  associations  in  any  given  area  the  temperature  condi- 
tions are  nearly  uniform.  Usually  plants  work  only  at 
temperatures  between  32°  and  122°  Fahr.,  but  for  each 
plant  there  is  its  own  range  of  temperature,  sometimes 
extensive,  sometimes  restricted.  Even  in  plant  associations, 
however,  the  effect  of  the  heat  factor  may  be  noted  in  the 
succession  of  plants  through  the  working  season,  spring 
plants  being  very  different  from  summer  and  autumn 
plants. 

Soil. — The  great  importance  of  this  factor  is  evident, 
even  in  water  plants,  for  the  soil  of  the  drainage  area  deter- 
mines the  materials  carried  by  the  water.  Soil  is  to  be 
considered  both  as  to  its  chemical  composition  and  its 
physical  properties,  the  latter  chiefly  in  reference  to  its 
disposition  toward  water.  Soils  vary  greatly  in  the  power 
of  receiving  and  retaining  water,  sand  having  a  high  recep- 
tive and  low  retentive  power,  and  clay  just  the  reverse, 
and  these  factors  have  large  effect  upon  vegetation. 

Light. — All  green  plants  can  not  receive  the  same  amount 
of  light.     Hence  some  of  them  have  learned  to  live  with  a 


PLANT  ECOLOGY 


315 


less  amount  than  others,  and  are  "shade  plants"  as  dis- 
tinct from  "  light  plants."  In  forests  and  thickets  many 
of  these  shade  plants  are  to  be  seen  which  would  find  an 
exposed  situation  hard  to  endure.  In  almost  every  associa- 
tion, therefore,  plants  are  arranged  in  strata,  dependent 
upon  the  amount  of  light  they  receive,  and  the  number  of 
these  strata  and  the  plants  characterizing  each  stratum  are 
important  factors  to  note. 

Wind. — This  is  an  important  factor  in  regions  where 
there  are  strong  prevailing  winds.  Wind  has  a  drying 
effect  and  increases  the  transpiration  of  plants,  tending  to 
impoverish  them  in  water.  In  such  conditions  only  those 
plants  can  live  which  are  well  adapted  to  regulate  trans- 
piration. 

The  above  five  factors  are  among  the  most  important, 
but  no  single  factor  determines  an  association.  As  each 
factor  has  a  large  possible  range,  the  combinations  of  fac- 
tors may  be  very  numerous,  and  it  is  these  combinations 
which  determine  associations.  For  convenience,  however, 
associations  are  usually  grouped  on  the  basis  of  the  water 
factor,  at  least  three  great  groups  being  recognized. 

179.  Hydrophyte  associations.— These  are  associations  of 
water  plants,  the  water  factor  being  so  conspicuous  that  the 
plants  are  either  submerged  or  standing  in  water.  A  plant 
completely  exposed  to  water,  submerged,  or  floating,  may 
be  taken  to  illustrate  the  usual  adaptations.  The  epi- 
dermal walls  are  thin,  so  that  water  may  be  absorbed 
through  the  whole  surface ;  hence  the  root  system  is  very 
commonly  reduced  or  even  wanting  ;  and  hence  the  water- 
conducting  tissues  (xylem)  are  feebly  developed.  The  tis- 
sues for  mechanical  support  (stereome)  are  feebly  devel- 
oped, the  plant  being  sustained  by  the  buoyant  power  of 
water.  Such  a  plant,  although  maintaining  its  form  in 
water,  collapses  upon  removal.  Very  common  also  is  the 
development  of  conspicuous  air  passages  for  internal  aera- 
tion and  for  increasing  buoyancy  ;  and  sometimes  a  special 


316  PLANT  STRUCTURES 

buoyancy  is  provided  for  by  the  development  of  bladder- 
like  floats. 

Conspicuous  among  hydrophyte  associations  may  be 
mentioned  the  following :  (1)  Free-swimming  associations, 
in  which  the  plants  are  entirely  sustained  by  water,  and  are 
free  to  move  either  by  locomotion  or  by  water  currents. 
Here  belong  the  "plankton  associations,"  consisting  of 
minute  plants  and  animals  invisible  to  the  naked  eye, 
conspicuous  among  the  plants  being  the  diatoms ;  also  the 
"  pond  associations,"  composed  of  algae,  duckweeds,  etc., 
which  float  in  stagnant  or  slow-moving  waters. 

(2)  Pondweed  associations,  in  which  the  plants  are  an- 
chored, but  their  bodies  are  submerged  or  floating.  Here 
belong  the  "rock  associations,"  consisting  of  plants  an- 
chored to  some  firm  support  under  water,  the  most  conspic- 
uous forms  being  the  numerous  fresh-water  and  marine 
algae,  among  which  there  are  often  elaborate  systems  of 
holdfasts  and  floats.  The  "  loose-soil  associations  "  are  dis- 
tinguished by  imbedding  their  roots  or  root-like  processes 
in  the  mucky  soil  of  the  bottom  (Figs.  281,  282).  The  wa- 
ter lilies  with  their  broad  floating  leaves,  the  pondweeds  or 
pickerel  weeds  with  their  narrow  submerged  leaves,  are 
conspicuous  illustrations,  associated  with  which  are  algae, 
mosses,  water  ferns,  etc. 

(3)  Swamp  associations,  in  which  the  plants  are  rooted 
in  water,  or  in  soil  rich  in  water,  but  the  leaf-bearing  stems 
rise  above  the  surface.  The  conspicuous  swamp  associations 
are  "reed  swamps,"  characterized  by  bulrushes,  cat-tails 
and  reed-grasses  (Figs.  283,  284),  tall  wand-like  Monocoty- 
ledons, usually  forming  a  fringe  about  the  shallow  margins 
of  small  lakes  and  ponds  ;  "  swamp-moors,"  the  ordinary 
swamps,  marshes,  bogs,  etc.,  and  dominated  by  coarse 
sedges  and  grasses  (Fig.  282) ;  "  swamp-thickets,"  consist- 
ing of  willows,  alders,  birches,  etc. ;  "  sphagnum-moors,"  in 
which  sphagnummoss  predominates,  and  is  accompanied  by 
numerous  peculiar  orchids,  heaths,  carnivorous  plants,  etc. ; 


39 


PLANT   ECOLOGY 


319 


"swamp-forests,"  which  are  largely  coniferous,  tamarack 
(larch),  pine,  hemlock,  etc.,  prevailing. 


180.  Xerophyte  associations. — These  associations  are  ex- 
posed  to  the  other  extreme  of  the  water  factor,  and  are  com- 
posed  of  plants  adapted  to  dry  air  and  soil.     To  meet  these 


320 


PLANT   STRUCTURES 


drought  conditions  numerous  adaptations  have  been  de- 
veloped and  are  very  characteristic  of  xerophytic  plants. 
Some  of  the  conspicuous  adaptations  are  as  follows :  peri- 


odic reduction  of  surface,  annuals  bridging  over  a  period 
of  drought  in  the  form  of  seeds,  geophilous  plants  also  dis- 
appearing from  the  surface  and  persisting  in  subterranean 


■Am 


324  PLANT   STRUCTURES 

parts,  deciduous  trees  and  shrubs  dropping  their  leaves, 
etc.  ;  temporary  reduction  of  surface,  the  leaves  rolling  up 
or  folding  together  in  various  ways ;  profile  position,  the 
leaves  standing  edgewise  and  not  exposing  their  flat  sur- 
faces to  the  most  intense  light ;  motile  leaves  which  can 
shift  their  position  to  suit  their  needs  ;  small  leaves,  a  very 
characteristic  feature  of  xerophytic  plants  ;  coverings  of 
hair ;  dwarf  growth ;  anatomical  adaptations,  such  as 
cuticle,  palisade  tissue,  etc.  Probably  the  most  conspicu- 
ous adaptation,  however,  is  the  organization  of  "water- 
reservoirs,"  which  collect  and  retain  the  scanty  water  sup- 
ply, doling  it  out  as  the  plant  needs  it. 

Some  of  the  prominent  associations  are  as  follows : 
"rock-associations,"  composed  of  plants  living  upon  exposed 
rock  surfaces,  Avails,  fences,  etc.,  notably  lichens  and  mosses ; 
"  sand  associations,"  including  beaches,  dunes,  and  sandy 
fields ;  "  shrubby  heaths,"  characterized  by  heath  plants ; 
"  plains,"  the  great  areas  of  dry  air  and  wind  developed  in 
the  interiors  of  continents;  "cactus  deserts,"  still  more 
arid  areas  of  the  Mexican  region,  where  the  cactus,  agave, 
yucca,  etc.,  have  learned  to  live  by  means  of  the  most  ex- 
treme xerophytic  modifications;  " tropical  deserts,"  where 
xerophytic  conditions  reach  their  extreme  in  the  combina- 
tion of  maximum  heat  and  minimum  water ;  "  xerophyte 
thickets,"  the  most  impenetrable  of  all  thicket-growths, 
represented  by  the  "  chaparral "  of  the  Southwest,  and  the 
"  bush  "  and  "  scrub  "  of  Africa  and  Australia ;  "  xero- 
phyte forests,"  also  notably  coniferous.  (See  Figs.  285, 
286,  287.) 

181.  Mesophyte  associations. — Mesophytes  make  up  the 
common  vegetation,  the  conditions  of  moisture  being  me- 
dium, and  the  soil  fertile.  This  is  the  normal  plant  condi- 
tion, and  is  the  arable  condition — that  is,  best  adapted  for 
the  plants  which  man  seeks  to  cultivate.  If  a  hydrophytic 
area  is  to  be  cultivated,  it  is  drained  and  made  mesophytic  ; 
if  a  xerophytic  area  is  to  be  cultivated,  it  is  irrigated  and 


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PLANT  ECOLOGY 


327 


made  mesophytic.  As  contrasted  with  hydrophyte  and 
xerophyte  associations,  the  mesophyte  associations  are  far 
richer  in  leaf  forms  and  in  general  luxuriance.  The  arti- 
ficial associations  which  have  been  formed  under  the  influ- 
ence of  man,  through  the  introduction  of  weeds  and  culture 
plants,  are  all  mesophytic. 

Among  the  mesophyte  grass  and  herb  associations  are 
the  "  arctic  and  alpine  carpets,"  so  characteristic  of  high 
latitudes  and  altitudes  where  the  conditions  forbid  trees, 
shrubs,  or  even  tall  herbs ;  "  meadows,"  areas  dominated  by 
grasses,  the  prairies  being  the  greatest  meadows,  where 
grasses  and  flowering  herbs  are  richly  displayed;  "pas- 
tures," drier  and  more  open  than  meadows. 

Among  the  woody  mesophyte  associations  are  the  "thick- 
ets," composed  of  willow,  alder,  birch,  hazel,  etc.,  either 
pure  or  forming  a  jungle  of  mixed  shrubs,  brambles,  and 
tall  herbs  ;  "  deciduous  forests,"  the  glory  of  the  temperate 
regions,  rich  in  forms  and  foliage  display,  with  annual  fall 
of  leaves,  and  exhibiting  the  remarkable  and  conspicuous 
phenomenon  of  autumnal  coloration;  " rainy  tropical  for- 
ests," in  the  region  of  trade  winds,  heavy  rainfalls,  and 
great  heat,  where  the  world's  vegetation  reaches  its  climax, 
and  where  in  a  saturated  atmosphere  gigantic  jungles  are 
developed,  composed  of  trees  of  various  heights,  shrubs  of 
all  sizes,  tall  and  low  herbs,  all  bound  together  in  an  inex- 
tricable tangle  by  great  vines  or  lianas,  and  covered  by  a 
luxuriant  growth  of  numerous  epiphytes.  (See  Figs.  288, 
289.) 


GLOSSARY 


[The  definitions  of  a  glossary  are  often  unsatisfactory.  It  is  much  better  to  con- 
sult the  fuller  explanations  of  the  text  by  means  of  the  index.  The  following  glos- 
sary includes  only  frequently  recurring  technical  terms.  Those  which  are  found  only 
in  reasonably  close  association  with  their  explanation  are  omitted.  The  number  fol- 
lowing each  definition  refers  to  the  page  where  the  term  will  be  found  most  fully 
defined.] 


Actinomorphic  :  applied  to  a  flower  in  which  the  parts  in  each  set  are 

similar ;  regular.     228. 
Akene  :  a  one-seeded  fruit  which  ripens  dry  and  seed-like.    212. 
Alternation  of  generations  :   the  alternation  of  gametophyte  and 

sporophyte  in  a  life  history.     94. 
Anemophilous  :  applied  to  flowers  or  plants  which  use  the  wind  as  agent 

of  pollination.     181. 
Anisocarpic  :  applied  to  a  flower  whose  carpels  are  fewer  than  the  other 

floral  organs.     268. 
Anther  :  the  sporangium-bearing  part  of  a  stamen.     197. 
Antheridium  :  the  male  organ,  producing  sperms.     16. 
Antipodal  cells  :  in  Angiosperms  the  cells  of  the  female  gametophyte 

at  the  opposite  end  of  the  embryo-sac  from  the  egg-apparatus. 

205. 
Apetalous  :  applied  to  a  flower  with  no  petals.     221. 
Apocarpous  :  applied  to  a  flower  whose  carpels  are  free  from  one  an- 
other.    226. 
Archegonium  :  the  female,  egg-producing  organ  of  Bryophytes,  Pteri- 

dophytes,  and  Gymnosperms.     100. 
Arciiesporium  :  the  first  cell  or  group  of  cells  in  the  spore-producing 

series.     102. 
Ascocarp  :  a  special  case  containing  asci.     58. 
Ascospore  :  a  spore  formed  within  an  ascus.     59. 
Ascus:  a  delicate  sac  (mother-cell)  within  which  ascospores  develop. 

59. 
Asexual  spore  :  one  produced  usually  by  cell-division,  at  least  not  by 

cell-union.     9. 

329 


330  GLOSSARY 

Calyx  :  the  outer  set  of  floral  leaves.    221. 

Capsule  :  in  Bryophytes  the  spore- vessel ;  in  Angiosperms  a  dry  fruit 

which  opens  to  discharge  its  seeds.     98,  211. 
Carpel  :  the  megasporophyll  of  Sperraatophytes.     178. 
Chlorophyll  :  the  green  coloring  matter  of  plants.    5. 
Chloroplast  :  the  protoplasmic  body  within  the  cell  which  is  stained 

green  by  chlorophyll.     7. 
Columella  :  in  Bryophytes  the  sterile  tissue  of  the  sporogonium  which 

is  surrounded  by  the  sporogenous  tissue.     106. 
Conidium  :  an  asexual  spore  formed  by  cutting  off  the  tip  of  the  sporo- 

phore,  or  by  the  division  of  hyphae.    58. 
Conjugation  :  the  union  of  similar  gametes.     15. 
Corolla  :  the  inner  set  of  floral  leaves.     221. 

Cotyledon  :  the  first  leaf  developed  by  an  embryo  sporophyte.     138. 
Cyclic  :  applied  to  an  arrangement  of  leaves  or  floral  parts  in  which 

two  or  more  appear  upon  the  axis  at  the  same  level,  forming  a  cycle, 

or  whorl,  or  verticil.     159. 

Dehiscence  :  the  opening  of  an  organ  to  discharge  its  contents,  as  in 
sporangia,  pollen-sacs,  capsules,  etc.     199. 

Dichotomous  :  applied  to  a  style  of  branching  in  which  the  tip  of  the 
axis  forks.     35. 

Dicecious  :  applied  to  plants  in  which  the  two  sex-organs  are  upon  dif- 
ferent individuals.     115. 

Dorsiventral  :  applied  to  a  body  whose  two  surfaces  are  differently 
exposed,  as  an  ordinary  thallus  or  leaf.     109. 

Egg  :  the  female  gamete.     16. 

Egg-apparatus  :  in  Angiosperms  the  group  of  three  cells  in  the  embryo- 
sac  composed  of  the  egg  and  the  two  synergids.     204. 
Elater  :  in  Liverworts  a  spore-mother-cell  peculiarly  modified  to  aid 

in  scattering  the  spores.     103. 
Embryo  :  a  plant  in  the  earliest  stages  of  its  development  from  the 

spore.    137. 
Embryo-sac  :  the  megaspore  of  Spermatophytes,  which  later  contains 

the  embryo.     178. 
Endosperm  :  the  nourishing  tissue  developed  within  the  embryo-sac,  and 

thought  to  represent  the  female  gametophyte.     180. 
Endosperm  nucleus  :  the  nucleus  of  the  embryo-sac  which  gives  rise  to 

the  endosperm.     205. 
Entomophilous  :  applied  to  flowers  or  plants  which  use  insects  as  agents 

of  pollination.     196. 


GLOSSARY  331 

Epigynous  :  applied  to  a  flower  whose  outer  parts  appear  to  arise  from 

the  top  of  the  ovary.    225. 
Eusporangiate  :  applied  to  those  Pteridophytes  and  Spermatophytes 

whose  sporangia  develop  from  a  group  of  epidermal  and  deeper 

cells.     157. 

Family  :  a  group  of  related  plants,  usually  comprising  several  genera. 

236. 
Fertilization  :  the  union  of  sperm  and  egg.     16. 
Filament  :  the  stalk-like  part  of  a  stamen.     197. 
Fission:    cell  -  division   which    includes    the    wall    of    the    old    cell. 

10. 
Foot  :  in  Bryophytes  the  part  of  the  sporogonium  imbedded  in  the 

gametophore ;  in  Pteridophytes  an  organ  of  the  sporophyte  embryo 

to  absorb  from  the  gametophyte.     98,  138. 

Gametangium  :  the  organ  within  which  gametes  are  produced.     11. 

Gamete  :  a  sexual  cell,  which  by  union  with  another  produces  a  sexual 
spore.     10. 

Gametophore  :  a  special  branch  which  bears  sex  organs.    98. 

Gametophyte  :  in  alternation  of  generations,  the  generation  which  bears 
the  sex  organs.     97. 

Generative  cell:  in  Spermatophytes  the  cell  of  the  male  gameto- 
phyte (within  the  pollen  grain)  which  gives  rise  to  the  male 
cells.     180. 

Genus  :  a  group  of  very  closely  related  plants,  usually  comprising  sev- 
eral species.     237. 

Haustorium  :  a  special  organ  of  a  parasite  (usually  a  fungus)  for  ab- 
sorption.   50. 

Heterogamous  :  applied  to  plants  whose  pairing  gametes  are  un- 
like.    15. 

Heterosporous  :  applied  to  those  higher  plants  whose  sporophyte  pro- 
duces two  forms  of  asexual  spores.     151. 

Homosporous:  applied  to  those  plants  whose  sporophyte  produces  simi- 
lar asexual  spores.     151. 

Host  :  a  plant  or  animal  attacked  by  a  parasite.    48. 

Hypha  :  an  individual  filament  of  a  mycelium.     49. 

Hypocotyl  :  the  axis  of  the  embryo  sporophyte  between  the  root-tip  and 
the  cotyledons.     209. 

Hypogynous  :  applied  to  a  flower  whose  outer  parts  arise  from  beneath 
the  ovary.    224. 


332  GLOSSARY 

Indusium  :  in  Ferns  a  flap-like  membrane  protecting  a  sorus.     143. 

Inflorescence  :  a  flower-cluster.    230. 

Insertion  :  the  point  of  origin  of  an  organ.     224. 

Integument  :  in  Spermatophytes  a  membrane  investing  the  nucellus. 

178. 
Involucre  :  a  cycle  or  rosette  of  bracts  beneath  a  flower-cluster,  as  in 

Umbellifers  and  Composites.    275. 
IsoCARPic  :  applied  to  a  flower  whose  carpels  equal  in  number  the  other 

floral  organs.     268. 
Isogamous  :  applied  to  plants  whose  pairing  gametes  are  similar.     15. 

Leptosporangiate  :  applied  to  those  Ferns  whose  sporangia  develop 
from  a  single  epidermal  cell.     157. 

Male  cell  :  in  Spermatophytes  the  fertilizing  cell  conducted  by  the 

pollen-tube  to  the  egg.     180. 
Megasporangium  :  a  sporangium  which  produces  only  megaspores.   152. 
Megaspore  :  in  heterosporous  plants  the  large  spore  which  produces  a 

female  gametophyte.     152. 
Megasporophyll  :  a  sporophyll  which  produces  only  megasporangia. 

152. 
Mesophyll  :  the  tissue  of  a  leaf  between  the  two  epidermal  layers  which 

usually  contains  chloroplasts.     141. 
Microsporangium  :   a  sporangium  which  produces  only  microspores. 

152. 
Microspore  :  in  heterospoi-ous  plants  the  small  spore  which  produces  » 

male  gametophyte.     152. 
Microsporophyll  :  a  sporophyll  which  produces  only  microsporangia- 

152. 
Micropyle:  the  passageway  to  the  nucellus  left  by  the  integument. 

178. 
Moncecious  :  applied  to  plants  in  which  the  two  sex  organs  are  upon 

the  same  individual.     115. 
Monopodial  :  applied  to  a  style  of  branching  in  which  the  branches 

arise  from  the  side  of  the  axis.     35. 
Mother  cell  :  usually  a  cell  which  produces  new  cells  by  internal  divi- 
sion.   9. 
Mycelium  :  the  mat  of  filaments  which  composes  the  working  body  of 

a  fungus.     49. 

Naked  flower  :  one  with  no  floral  leaves.    222. 
Nucellus  :  the  main  body  of  the  ovule.     178. 


GLOSSARY 


333 


Oogonium  :  the  female,  egg-producing  organ  of  Thallophytes.     16. 

Oosphere  :  the  female  gamete,  or  egg.    16. 

Oospore  :  the  sexual  spore  resulting  from  fertilization.     16. 

Ovary  :  in  Angiosperms  the  bulbous  part  of  the  pistil,  which  contains 

the  ovules.     199. 
Ovule  :  the  megasporangium  of  Spermatophytes.     178. 

Pappus  :  the  modified  calyx  of  the  Composites.    278. 

Parasite  :  a  plant  which  obtains  food  by  attacking  living  plants  or  ani- 
mals.   48. 

Pentacyclic  :  applied  to  a  flower  whose  four  floral  organs  are  in  five 
cycles,  the  stamens  being  in  two  cycles.    268. 

Perianth  :  the  set  of  floral  leaves  when  not  differentiated  into  calyx 
and  corolla.    221. 

Periqynous  :  applied  to  a  flower  whose  outer  parts  arise  from  a  cup 
surrounding  the  ovary.     225. 

Petal  :  one  of  the  floral  leaves  which  make  up  the  corolla.    221. 

Photosynthesis  :  the  process  by  which  chloroplasts,  aided  by  light, 
manufacture  carbohydrates  from  carbon  dioxide  and  water.    84. 

Pistil  :  the  central  organ  of  the  flower,  composed  of  one  or  more  car- 
pels.   200. 

Pistillate  :  applied  to  flowers  with  carpels  but  no  stamens.    218. 

Pollen  :  the  microspores  of  Spermatophytes.     174. 

Pollen-tube  :  the  tube  developed  from  the  wall  of  the  pollen  grain 
which  penetrates  to  the  egg  and  conducts  the  male  cells.     180. 

Pollination  :  the  transfer  of  pollen  from  anther  to  ovule  (in  Gymno- 
sperms)  or  stigma  (in  Angiosperms).     181. 

Polypetalous  :  applied  to  flowers  whose  petals  are  free  from  one  an- 
other.   227. 

Prothallium  :  the  gametophyte  of  Ferns.    130. 

Protonema  :  the  thallus  portion  of  the  gametophyte  of  Mosses.    98. 

Radial  :  applied  to  a  body  with  uniform  exposure  of  surface,  and  pro- 
ducing similar  organs  about  a  common  center.    120. 

Receptacle  :  in  Angiosperms  that  part  of  the  stem  which  is  more  or 
less  modified  to  support  the  parts  of  the  flower.     222. 

Rhizoid  :  a  hair-like  process  developed  by  the  lower  plants  and  by  inde- 
pendent gametophytes  to  act  as  a  holdfast  or  absorbing  organ,  or 
both.     109. 

Saprophyte  :  a  plant  which  obtains  food  from  the  dead  bodies  or  body 
products  of  plants  or  animals.    48. 
40 


334  GLOSSARY 

Scale  :  a  leaf  without  chlorophyll,  and  usually  reduced  in  size. 
161. 

Sepal  :  one  of  the  floral  leaves  which  make  up  the  calyx.    221. 

Seta  :  in  Bryophytes  the  stalk-like  portion  of  the  sporogoniuni.    98. 

Sexual  spore  :  one  produced  by  the  union  of  gametes.     10. 

Species  :  plants  so  nearly  alike  that  they  all  might  have  come  from  a 
single  parent.    237. 

Sperm  :  the  male  gamete.     16. 

Spiral  :  applied  to  an  arrangement  of  leaves  or  floral  parts  in  which 
no  two  appear  upon  the  axis  at  the  same  level ;  often  called  alter- 
nate.    193. 

Sporangium  :  the  organ  within  which  asexual  spores  are  produced  (ex- 
cept in  Bryophytes).     10. 

Spore  :  a  cell  set  apart  for  reproduction.    9. 

Sporogonium  :  the  leafless  sporophyte  of  Bryophytes.    98. 

Sporophore  :  a  special  branch  bearing  asexual  spores.    49. 

Sporophyll  :  a  leaf  set  apart  to  produce  sporangia.     145. 

Sporophyte  :  in  alternation  of  generations,  the  generation  which  pro- 
duces the  asexual  spores.     97. 

Stamen  :  the  microsporophyll  of  Spermatophytes.     174. 

Staminate  :  applied  to  a  flower  with  stamens  but  no  carpels.    218. 

Stigma  :  in  Angiosperms  that  portion  of  the  carpel  (usually  of  the  style) 
prepared  to  receive  pollen.     199. 

Stoma  (pi.  Stomata)  :  an  epidermal  organ  for  regulating  the  communi- 
cation between  green  tissue  and  the  air.     141. 

Strobilus  :  a  cone-like  cluster  of  sporophylls.     161. 

Style  :  the  stalk-like  prolongation  from  the  ovary  which  bears  the 
stigma.     199. 

Suspensor  :  in  heterosporous  plants  an  organ  of  the  sporophyte  embryo 
which  places  it  in  a  more  favorable  position  in  reference  to  food 
supply.     168. 

Symbiont  :  an  organism  which  enters  into  the  condition  of  symbio- 
sis.    79. 

Symbiosis  :  usually  applied  to  the  condition  in  which  two  different 
organisms  live  together  in  intimate  and  mutually  helpful  rela- 
tions.    79. 

Sympetalous  :  applied  to  a  flower  whose  petals  have  coalesced. 
227. 

Syncarpous  :  applied  to  a  flower  whose  carpels  have  coalesced. 
226. 

Synergid  :  in  Angiosperms  one  of  the  pair  of  cells  associated  with  the 
egg  to  form  the  egg-apparatus.    204. 


GLOSSARY  335 

Testa  :  the  hard  coat  of  the  seed.     184. 

Tetracyclic  :  applied  to  a  flower  whose  four  floral  organs  are  in  four 

cycles.     268. 
Tetrad  :  a  group  of  four  spores  produced  by  a  mother-cell.     103. 

Zoospore  :  a  motile  asexual  spore.    10. 

Zygomorphic  :  applied  to  a  flower  in  which  the  parts  in  one  or  more 

sets  are  not  similar  ;  irregular.    229. 
Zygote  :  the  sexual  spore  resulting  from  conjugation.     15. 


INDEX  TO   PLANT   STRUCTURES 


[The  italicized  numbers  indicate  that  the  subject  is  illustrated  upon  the  page  cited. 
In  such  case  the  subject  may  be  referred  to  only  in  the  illustration,  or  it  may  be 
referred  to  also  in  the  text.] 


Absorption,  299. 
Acacia,  265. 
Aconitum,  261. 
Acorus,  2 19,  243. 
Aetinomorphy,  228. 
Adder's  tongue  :  see  Ophioglossum. 
Adiantura,  143,  145. 
jEcidiomyeetes,  50,  62. 
^Ecidiospore,  66. 
iEcidium,  66. 
Agaricus,  68,  69. 
Agave,  247. 
Air  pore :  see  Stoma. 
Akene,  212,  213,  214,  276,  277. 
Alchemilla,  225. 
Alder :  see  Alnus. 
Algae,  4,  5,  17. 
Alisma,  210,  240. 
Almond :  see  Prunus. 
Alnus,  257. 

Alternation  of  generations,  94,  129. 
Amanita,  70. 
Amaryllidaceae,  247. 
Amaryllis   family :   see  Amarylli- 
daceae. 
Ambrosia,  279. 
Ament,  257. 
Anaptychia,  81,  82. 


Anemophilous,  181. 
Angiosperms,  173,  195,  217. 
Anisocarpae,  268. 
Annulus,  ISti,  146,  150. 
Anther,  196,  197,  199. 
Antheridium,  16,  99,  100,  112,  121, 

133,  134,  161,  166. 
Antherozoid,  16. 

Anthoceros,  104,  105,  111,  116,  118. 
Anthophytes,  172. 
Antipodal  cells,  202,  205,  208. 
Antirrhinum,  228,  275. 
Ant-plants,  90,  91. 
Apical  cell,  13 4. 
Apical  group,  283. 
Apium,  267. 
Apocarpy,  199.  222,  225. 
Apocynaceae,  271. 
Apocynum,  272. 
Apogamy,  131. 
Apospory,  132. 
Apothecium,  79,  81,  82. 
Apple :  see  Pirus. 
Aquilegia,  198. 
Araceae,  243. 
Araliaceae,  267. 
Araucaria,  190. 
Arbor  vitas :  see  Thuja. 
Arbutus,  198 :  see  Epigaea. 
Archegoniates,  101. 
337 


338 


INDEX 


Archegonium,  99, 100, 113, 114, 133, 

135,  161,  167,  179. 
Archesporium,  102,  104,  105,  146. 
Archichlamydea?,  255. 
Arctostaphylos,  269. 
Areolae,  111,  114. 
Arisaema,  243,  244. 
Arnica,  275,  276,  278. 
Aroids,  243. 
Artemisia,  279. 
Arum,  245. 
Ascoearp,  58,  59. 
Ascomycetes,  50,  57. 
Ascospore,  59. 
Aseus,  59. 
Asexual  spore,  9. 
Aspidium,  130,  136,  144. 
Assimilation,  302. 
Aster,  279. 
Astragalus,  265. 
Atherosperma,  198. 
Azalea,  270. 

B 

Bacillus,  76. 

Bacteria,  21,  75.  76. 

Balm :  see  Melissa. 

Banana,  140. 

Bark,  284,  289. 

Basidiomycetes,  50,  68 

Basidiospore,  69,  72. 

Basidium,  69,  71. 

Bean  :  see  Phaseolus. 

Bearberry :  see  Arctostaphylos. 

Beech,  256. 

Bellis,  279. 

Berberis,  198. 

Bidens,  278. 

Beggar-ticks,  213. 

Bignonia,  211. 

Birch.  256. 

Blackberry :  see  Rubus. 


Black  knot,  60. 
Black  mould,  52. 
Blasia,  116. 

Blueberry :  see  Vaccinium. 
Blue-green  algae,  6,  17. 
Blue  mould,  60. 
Boiet  is,  73,  74. 
Botrychium,  145,  149. 
Botrydium,  28. 
Box  elder,  234. 
Bracket  fungus,  72. 
Brake:  see  Pteris. 
Brassica,  261. 
Bryophytes,  2,  93,  172. 
Brown  algae,  6,  32. 
Bryum,  120,  124. 
Buckeye,  235. 
Butomus,  199. 

Buttercup :  see  Ranunculus. 
Buttercup  family  :  see   Ranuncu- 
laceae. 

C 

Cabbage :  see  Brassica. 

Calamus :  see  Acorus. 

Calla-lily,  243. 

Callithamnion,  43. 

Callophyllis,  39. 

Call  una,  270. 

Calopogon,  249. 

Caltha,  260. 

Cal  yean  thus,  226,  261. 

Calypso,  249. 

Calyptra,  102,  125. 

Calyptrogen,  293. 

Calyx,  220,  221. 

Cambium,  285,  287,  288. 

Capsella,  209,  293. 

Capsule,  98, 123, 125, 126,  211,  212. 

Caraway :  see  Carum. 

Carbohydrate,  302. 

Carbon  dioxide,  83. 


INDEX 


339 


Carnivorous  plants,  92. 

Carpel,  177,  178,  199,  219,  220. 

Carpinus,  217,  258. 

Carpospore,  44,  45. 

Carrot :  see  Daucus. 

Carum,  267. 

Cassia,  265. 

Cassiope,  269. 

Castilleia,  275. 

Catkin,  257. 

Catnip :  see  Nepeta. 

Cat-tail :  see  Typha. 

Cattleya,  254. 

Caulicle,  209. 

Cauline,  166. 

Cedar  apple,  67,  68. 

Celery :  see  Apium. 

Cell,  6,  7. 

Cellulose,  7. 

Cercis,  265. 

Chalazogamy,  258,  259. 

Characeae,  46. 

Chemotropism,  307. 

Cherry :  see  Primus. 

Chestnut,  256. 

Chlorophyceae,  6,  21. 

Chlorophyll,  5,  83. 

Chloroplast,  7,  8. 

Chrysanthemum,  279. 

Cilia,  10. 

Circinate,  136,  143. 

Cladophora,  25. 

Clavaria,  73. 

Climbing  fern  :  see  Lygodium. 

Closed  bundle,  290. 

Clover :  see  Trifolium. 

Club  mosses,  162. 

Cnicus,  278. 

Cocklebur :  see  Xanthium. 

Coenocyte,  27. 

Coleochaete,  106,  107. 

Collateral  bundle,  287. 


Collenchyma,  284. 

Columella,  104,  105,  106,  126. 

Compass  plant :  see  Silphium. 

Composite,  275. 

Composites,  275,  276,  277,  278. 

Concentric  bundle,  292. 

Conferva  forms,  22. 

Conidia,  58,  60. 

Conifers,  191,  282. 

Conium,  267. 

Conjugate  forms,  31. 

Conjugation,  15. 

Connective,  196. 

Conocephalus,  111. 

Convolvulacete,  271. 

Convolvulus  forms,  270- 

Convolvulus,  273. 

Coprinus,  70. 

Coral  fungus,  73,  74. 

Coreopsis,  278. 

Coriandrum,  267. 

Cork,  284. 

Corn,  216,  282,  290. 

Cornaceae,  267. 

Corolla,  220,  221. 

Cortex,  283,  284,  288. 

Cotton,  206. 

Cotyledon,  137,  138,  168,  184,  209, 

210,  216,  217. 
Cranberry:  see  Vaccinium. 
Crataegus,  262. 
Crocus,  249. 
Crucifer,  262. 
Cruciferae,  262. 
Cryptogams,  172. 
Cunila,  274. 
Cup  fungus,  60,  61. 
Cupule,  112,  114. 
Cyanophyceae.  6  17. 
Cycads,  185,  186,  187,  189. 
Cyclic,  159,  193. 
Cyperaceae,  241. 


340 


INDEX 


Cypripedium,  249,  253. 
Cystocarp,  43,  44- 
Cystopteris,  78,  144. 
Cytoplasm,  7. 

D 

Daisy :  see  Bellis. 

Dandelion :  see  Taraxacum. 

Dasya,  40. 

Datura,  197. 

Daucus,  266,  267. 

Dead-nettle,  228. 

Definitive  nucleus:  see  Endosperm 

nucleus. 
Dehiscence,  198,  199. 
Delphinium,  260,  261. 
Dermatogen,  283. 
Desmids,  31,  32. 
Desmodium,  308. 
Diatoms,  45. 
Dichotomous,  35. 
Dicotyledons,  208,  233,  254,  282. 
Differentiation,  3,  280. 
Dogbane:  see  Apocynum. 
Dog-tooth  violet :  see  Erythronium. 
Dogwood  family :  see  Cornaceae. 
Dorsiventral,  109. 
Downy  mildew,  55. 
Drupe,  264. 
Digestion,  302. 
Dioecious,  115. 
Disk,  276,  277. 
Dodder,  86. 

E 

Ear-fungus,  74. 
Easter  lily,  221. 
Ecology,  297,  311. 
Economic  botany,  297. 
Ectocarpus,  33. 
Edogonium,  22,  23. 
Egg,  16,  202,  204,  205,  206. 


Egg-apparatus,  204,  205,  206. 

Elater,  103,  113,  118. 

Elm :  see  Ulmus. 

Embryo,  137, 167, 168, 170, 183, 207, 

208,  209,  210,  211. 
Embryo-sac,  178,  179, 201, 203, 208. 
Endosperm,  179,  180,  207,  208, 211. 
Endosperm  nucleus,  202,  205. 
Entomophilous,  196. 
Epidermis,  141,  142,  191,  283,  284, 

295. 
Epigaea,  269. 
Epigyny,  224,  225. 
Epilobium,  212. 
Epiphyte,  157. 
Equisetales,  159. 
Equisetum,  159,  160, 161. 
Ergot,  60,  61. 
Erica,  270. 
Ericaceae,  268. 
Erigenia,  267. 
Erythronium,  250. 
Eusporangiate,  157. 
Evolution,  3. 

F 

Fennel :  see  Fceniculum. 

Ferns,  155,  156. 

Fertilization,  16,  181,  206,  207. 

Festuca,  240. 

Figwort    family :    see    Scrophula- 

riaceae. 
Filament,  8,  196,  197. 
Filicales,  155. 
Fireweed :  see  Epilobium. 
Fission,  10. 
Flax :  see  Linum. 
Floral  leaves,  218. 
Florideae,  38. 
Flower,  218. 
Flowering  plants,  172. 
Fceniculum,  267. 


INDEX 


341 


Foliar,  166. 

Food,  83, 299. 

Foot,  98,  102,  137,  138,  168. 

Fragaria,  214,  227,  262. 

Fruit,  211,  212,  213,  214,  215. 

Fucus,  35,  37. 

Funaria,  99,  102,  121,  124, 125, 126. 

Fungi,  4,  48. 

G 

Gametangium,  11. 

Gamete,  10,  12. 

Gametophore,  98,  112,  120,  124. 

Garnetophyte,  97, 107, 132, 134, 161, 

166,  167,  176,  179,  180,  201,  203, 

204,  205. 
Gaultheria,  270. 
Gaylussacia,  269. 
Gemma,  112,  114. 
Generative  cell,  180,  201. 
Gentianaceae,  271. 
Geophilous,  246. 
Geotropism,  305. 
Gerardia,  275. 
Germination,  187,  214. 
Gigartina,  38. 
Gills,  71. 
Ginkgo,  191. 
Gladiolus,  249,  251. 
Gleditschia,  236,  265. 
Gloeocapsa,  17,  18. 
Glume,  241. 

Goldenrod :  see  Solidago. 
Gonatonema,  31. 
Gramineae,  241. 
Green  algae,  6.  21. 
Green  plants,  83. 
Green  slimes,  20. 
Grimmia,  126. 
Growth  movement,  304. 
Growth  ring,  234. 
Grain,  241. 


Grasses,  240. 

Grass  family :  see  Gramineae. 
Gymnosperms,  171,  173,  195. 
Gymnosporangium,  67. 

H 

Habenaria,  249,  252. 

Harebell,  228. 

Haustoria,  50. 

Hazel :  see  Carpinus. 

Heart-wood,  289. 

Heat,  314. 

Heath  family :  see  Ericaceae. 

Heaths,  268,  269,  270. 

HeliauLhus,  279,  285,  306. 

Heliotropism,  305. 

Hemiarcyria,  75. 

Hemlock :  see  Conium. 

Henbane :  see  Hyoscyamus. 

Hepaticae,  109. 

Heterocyst,  18. 

Heterogamy,  15. 

Heterospory,  151. 

Hickory,  256. 

Hippuris,  283. 

Homospory,  151. 

Honey  locust :  see  Gleditschia. 

Horehound :  see  Marrubium. 

Hornbeam :  see  Carpinus. 

Horsetail,  159. 

Host,  48. 

Huckleberry:  see  Gaylussacia. 

Hydnum,  73,  74. 

Hydra,  90. 

Hydrophytes,  6,  315. 

Hydrophytum,  91. 

Hydrotropism,  307. 

Hygroscopic  movement,  304. 

Hyoscyamus,  196. 

Hypha,  49. 

Hypocotyl,  184,  209,  216,  217. 


342 


INDEX 


Hypodermis,  284. 
Hypogyny,  224,  225. 
Hyssopus,  274. 

I 

Indigo :  see  Indigofera. 

Indigofera,  265. 

Indusium,  136,  143,  144. 

Inflorescence,  230. 

Insects  and  flowers,  90. 

Integument,  178,  179,  201, 202,  203. 

Involucre,  267,  275,  277. 

Ipomoea,  228,  270. 

Iridaceae,  247. 

Iris,  248,  251. 

Iris  family :  see  Iridaceae. 

Irritable  movement,  307. 

Isocarpae,  268. 

Isoetes,  169. 

Isogamy,  15. 


Japan  lily,  248. 

Jungermannia,  105,  115,  116,  117. 

Juniper,  194- 


K 


Kalmia.  270. 


Labiata?,  272. 

Labiates,  272. 

Lactuca,  279. 

Laminaria,  33,  34- 

Lamium,  274,  275. 

Larch  :  see  Larix. 

Larix,  192. 

Larkspur :  see  Delphinium. 

Laurel :  see  Kalmia. 

Lavandula,  275. 

Leaf,  141,  142,  295,  296,  311. 

Legumes,  250,  251,  264. 

Leguminosae,  264. 


Lemna,  201. 

Lepidozia.  117. 

Lepiota,  70. 

Leptosporaugiate,  157. 

Lettuce :  see  Lactuca. 

Leucanthemum,  279. 

Liatris,  278. 

Lichens,  77,  78,  79,  87. 

Life  relations,  311. 

Light,  314. 

Ligule.  168. 

Liliaceae,  246. 

Lilies,  245. 

Lilium,  203,  204,  205,  207,  224,  249, 

295. 
Lily:  see  Lilium. 
Lily  family :  see  Liliaceae. 
Linaria,  228,  275. 
Linum,  220. 
Liverworts,  109. 
Loculus,  200. 
Locust :  see  Robinia. 
Lotus,  264. 
Lupinus,  265. 
Lycopersicum,  275. 
Lycopodiales,  162. 
Lycopodium,  162,  163. 
Lygodium,  145. 
Lyonia,  269. 

M 

Macrospore,  152. 

Maidenhair  fern  :  see  Adiantum. 

Male  cell,  180,  181,  201,  206,  207. 

Maple,  212. 

Marasmius,  70. 

Marchantia,  104, 110,  111,  112,  113, 

114. 
Marguerite :  see  Leucanthemum. 
Marjoram  :  see  Origanum. 
Marrubium,  275. 
Marsh  marigold :  see  Caltha. 


INDEX 


343 


Marsilia,  158. 

Megasporangium,  152,  177,  179. 
Megaspore,  152,  165,  167,  179,  201, 

203. 
Megasporophyll,  152, 165,  177,  199. 
Melissa,  275. 
Mentha,  229, 274. 
Meristem,  281. 

Mesophyll,  141,  142,  191,  295. 
Mesophytes,  324. 
Mestome,  282. 

Micropyle,  17S,  201,  202,  206. 
Microspira,  76. 
Microspha^ra,  5S. 
Microsporangium,  152,  176,  197. 
Microspore,  152,  165,  166,  179,  197, 

201. 
Microsporophyll,  152, 165,  174,196, 

198. 
Midrib,  234. 
Mildews,  57. 
Mimosa,  265,  308,  309. 
Mint :  see  Mentha. 
Mint  family  :  see  Labiatae. 
Monocotyledons,  208,  232,  236, 289. 
Monoecious,  115. 
Monopodial,  35. 
Monotropa,  270. 
Moonwort :  see  Botrychium. 
Morels,  60,  62. 

Morning-glory :  see  Tpomcea. 
Morphology,  297. 
Mosses,  93,  119,  124. 
Mother  cell,  9. 
Mougeotia,  31. 
Movement,  303. 
Mucor,  49,  52,  53,  54,  55. 
Mullein :  see  Verbascum. 
Musci,  119. 
Mushrooms,  68. 

Mustard  family  :  see  Cruciferae. 
Mycelium,  49. 


Mycomycetes,  50. 
Mycorrhiza,  87,  88. 
Myristica,  214. 
Myrmecophytes,  90,  91. 
Myxomycetes,  74,  75. 

N 
Naias,  237. 
Narcissus,  247. 
Nemalion,  43. 
Nepeta,  275. 
Nicotiana,  227,  275. 
Nightshade  family  :  see  Solanaceae. 
Nostoc,  18. 

Nucellus,  178,  179,  201,  202,  203. 
Nucleus,  7. 
Nutation,  304. 
Nutmeg,  214. 
Nutrition,  3,  299. 
Nyctitropic  movement,  309. 
Nyrnphaeaceae,  261. 

O 
Oak,  255,  256. 

(Edogoniuin  :  see  Edogonium. 
Onoclea,  145,  147,  148. 
Oogonium,  16. 
Oosphere,  16. 
Oospore,  16,  101. 
Open  bundle,  287. 
Operculum,  122,  125. 
Ophioglossum,  145,  149. 
Orchidacea?,  249. 
Orchids,  249,  252,  253,  254. 
Orchid  family:  see  Orchidacea?. 
Origanum,  274. 
Ornithogalum,  247. 
Oscillatoria,  19. 
Osmunda,  145,  156. 
Ostrich  fern  :  see  Onoclea. 
Ovary,  199,  200,  202. 
Ovule,  178,  179,  201,  203. 


344 


INDEX 


Palisade  tissue,  142,  295. 

Palmaceae,  241. 

Palm  family  :  see  Palmaceae. 

Palms,  241,  gjg,  243. 

Papaveraceae,  261. 

Pappus,  276,  277,  27S. 

Parasites,  48,  85. 

Parenchyma,  280,  281,  282,  288. 

Parmelia,  79. 

Parsley:  see  Petroselinum. 

Parsley  family :  see  Umbellif  era?. 

Parsnip :  see  Pastinaca. 

Parthenogenesis,  52. 

Pastinaca,  267. 

Pathology,  297. 

Pea :  see  Pisum. 

Peach :  see  Prunus. 

Peach  curl,  60. 

Pea  family :  see  Leguminosae. 

Pear :  see  Pirus. 

Peat,  119. 

Pellaea,  146. 

Penicillium,  60. 

Pentacycke,  268. 

Pentstemon,  275. 

Peony,  220. 

Pepper.  211,  258. 

Pepper  family :  see  Piperaceae. 

Perianth,  219,  220,  221. 

Periblem,  283. 

Perigyny,  225,  226. 

Peristome,  126,  127. 

Peronospora,  55,  56. 

Petal,  220,  221. 

Petiole,  141. 

Petroselinum,  267. 

Phaeophyceae,  6,  32. 

Phanerogams,  172. 

Phaseolus,  216, 265. 

Phloem,  285,  287, 288, 290, 292, 294. 


Phlox,  228,  271. 

Photosyntax,  84. 

Photosynthesis,  84,  302. 

Phycomycetes,  50,  51. 

Physcia,  79. 

Physiology,  297. 

Picea,  179,  181, 182.      " 

Pileus,  71. 

Pine :  see  Pinus. 

Pineapple,  215. 

Pinus,  173,  175, 176, 177,  178, 181, 

183,  184,  188, 191,  286. 
Piperaceae,  258. 
Pirus,  225,  262,  263. 
Pistil,  199,  200,  219,  220. 
Pisum,  265. 
Pith,  285,  287,  288. 
Planococcus,  76. 
Plantaginacea?,  275. 
Plant  body,  6. 
Plant  associations,  313. 
Plasmodium,  74,  75. 
Plastid,  7,  8. 
Platycerium,  132. 
Plerome,  283. 
Pleurococcus,  21. 
Plum :  see  Prunus. 
Plumule,  210. 
Pod,  211,  212. 
Pogonia,  249. 
Polemoniacea?,  271. 
Polemonium,  271. 
Pollen,  174,  176,  197,  201. 
Pollen-tube,  179,  180,  181,  187,  202, 

206,  207. 
Pollination,  181. 
Polyembryony,  183. 
Polymorphism,  63. 
Polypetaly,  226. 
Polyporus,  71,  72. 
Polysiphonia,  44- 
Polytrichum,  96. 


INDEX 


345 


Pome,  263. 

Pondweeds,  237. 

Poplars,  255. 

Popowia,  198. 

Poppy,  261. 

Poppy  family :  see  Papaveracea?. 

Populus,  256. 

Pore-fungus,  72. 

Potamogeton,  237,  238. 

Potato :  see  Solatium. 

Potentilla,  225,  262. 

Proteid,  302. 

Prothallium,  130,  132,  134. 

Protococcus  forms,  22. 

Protonema,  95,  98. 

Protoplasm,  7. 

Prunus,  2 13,  262. 

Pseudomonas,  76. 

Pseudopodium,  105,  123,  124. 

Pteridophytes,  2,  128,  172,  291. 

Pteris,  133,  134,  135,  137,  141,  1&, 

143,  145,  281,  291,  292,  293. 
Ptilota,  42. 

Puccinia,  63,  64,  65,  66. 
Puff-balls,  68,  74. 
Pulvinus,  308. 

Q 
Quillwort :  see  Isoetes. 

R 

Rabdonia,  41- 
Radiate  bundle,  294. 
Radicle,  209. 
Radish,  120. 

Ragweed :  see  Ambrosia. 
Ranunculaceas,  261. 
Ranunculus,  222.  259. 
Raspberry :  see  Rubus. 
Rays,  275,  276. 
Receptacle.  222. 
Red  algae,  6,  38. 


Redbud :  see  Cercis. 

Redwood :  see  Sequoia. 

Reproduction,  3,  8,  309. 

Respiration,  302. 

Rheotropism,  307. 

Rhizoid,  109,  110,  134. 

Rhizophores,  I64. 

Rhododendron,  270,  271. 

Rhodophyceas,  6,  38. 

Riccia,  104,  110. 

Ricciocarpus,  110. 

Ricinus,  288. 

Robinia,  265. 

Root,  138,  217,  293,  294,  313. 

Root-cap,  293. 

Root-fungus,  87,  88. 

Root-hairs,  217,  300. 

Root-pressure,  300. 

Root-tubercles,  89. 

Rosacea?,  262. 

Rose  family :  see  Rosacea?. 

Rosin-weed :  see  Silphium. 

Rosmarinus,  275. 

Royal  fern  :  see  Osmunda. 

Rubus,  262. 

Rumex,  284. 

Rust,  62,  63,  64,  65,  66. 

S 
Sac-fungi,  57. 
Sage :  see  Salvia. 
Sage-brush :  see  Artemisia. 
Sagittaria,  208,  338. 
Salix,  219,  233,  256,  257. 
Salvia,  275. 
Salvinia,  158. 
Saprolegnia,  51,  52. 
Saprophyte,  48,  84. 
Sap-wood,  289. 
Sargassum,  35.  36. 
Saururus,  219,  258. 
Scales,  161. 


346 


INDEX 


Scapania,  116. 
Schizomycetes,  21. 
Schizophytes,  21. 
Sclerenchyma,  281,  282,  284,  285, 

288,  290,  291. 
Scouring  rush,  159. 
Scrophulariacea?,  275. 
Scutellaria,  275. 
Sedge  family:  see  Cyperaceae. 
Seed,  183, 184,  210,  211,  212,  214. 
Selaginella,  162,  164,  165,  166,  168. 
Sensitive  fern  :  see  Onoclea. 
Sensitive-plant :  see  Acacia. 
Sepal,  220,  221. 
Sequoia,  189. 
Seta,  98,  125. 
Sex,  12. 

Sexual  spore,  10. 
Shepherd's  purse :  see  Capsella. 
Shield  fern :  see  Aspidium. 
Shoot,  312. 

Sieve  vessels,  285,  286. 
Silphium,  279. 
Siphon  forms,  27. 
Siphonogams,  183. 
Siphonogamy,  183. 
Slime  moulds,  74,  75. 
Smut,  62. 

Snapdragon :  see  Antirrhinum. 
Soil,  314. 
Solanaeea^  275. 
Solanum,  198,  275. 
Solidago,  279. 
Solomon's  seal,  233. 
Sorus,  136,  143,  I44. 
Spadix,  244,  245. 
Spathe,  244,  245. 
Sperm,  16,  100,  133,  135,  162,  166, 

169, 187,  190. 
Spermatia,  43,  44. 
Spermatophytes,  2,  171,  172. 
Spermatozoid,  16. 


Sperm  mother  cell,  100. 

Sphagnum,  105,  106,  122, 123. 

Spike,  240. 

Spirasa,  262. 

Spiral,  193. 

Spirillum,  76. 

Spirogyra,  28,  29,  30. 

Spongy  tissue,  142. 

Sporangium,  10, 136,  143,  145,  150, 

157,  163, 179. 
Spore,  9. 
Sporidium,  65. 
Sporogenous  tissue,  103. 
Sporogonium,  98, 102,  104, 105, 106, 

125,  126. 
Sporophore,  4$,  50. 
Sporophyll,  145,  147,  148, 149,  174, 

176. 
Sporophyte,  97,  102, 137. 
Spruce  :  see  Picea. 
Stability  of  form,  298. 
Stamen,   174,  176,   196,   198,  219, 

220. 
Stele,  191,  283,  285. 
Stem,  139,  282,  289,  291,  312. 
Stemonitis,  75. 
Stereome,  282,  299. 
Sterile  tissue,  103. 
Sticta,  80. 
Stigma,  199,  202. 
Stomata,  141,  I42, 191,  295,  301. 
Strawberry :  see  Fragaria. 
Strobilus,  160,  161,  163,  165,  174, 

175,  176,  193,  194. 
Style,  199,  202. 
Substratum,  49. 
Sumach,  235. 

Sunflower :  see  Helianthus. 
Suspensor,     167,     168,     183,    209, 

210. 
Symbiont,  79,  86. 
Symbiosis,  79,  86. 


INDEX 


347 


Sy  in  petals?,  268. 
Synipetaly,  226,  ">27. 
Symplocarpus,  ',i43. 
Syncarpy,  199,  219,  225. 
Synergid,  202,  2'>4,  205,  206. 

T 

Tanacetum,  279. 
Tansy :  see  Tanacetum. 
Taraxacum,  213,  277,  278. 
Taxonomy,  297. 
Teleutospore,  64,  65. 
Tension  of  tissues,  298. 
Testa,  184,  211. 
Tetracycla\  268. 
Tetrad,  103. 
Tetraspore,  43. 
Teucrium,  230,  274,  275. 
Thallophytes,  2,  4,  172. 
Thermotropism,  307. 
Thistle :  see  Cnicus. 
Thorn  apple  :  see  Datura. 
Thuja,  193. 
Thymus,  274. 
Tickseed :  see  Coreopsis. 
Tissues,  280. 
Toad-flax :  see  Linaria. 
Toadstools,  68. 
Tobacco :  see  Nicotiana. 
Tomato :  see  Lycopersicum. 
Tracheae,  285,  286. 
Tracheids,  286. 
Transfer  of  water,  300. 
Transpiration,  301. 
Tree  fern,  140. 
Trichia,  75. 
Trichogyne,  43,  44. 
Trillium,  207,  246,  265. 
Truffles,  60. 
Turgidity,  298. 
Typha,  239,  240. 


D 

Umbel,  266,  267. 
Uinbelliferae,  266. 
Umbellifers,  266. 
Ulmus,  210,  256. 
Ulothrix,  12,  13,  22. 
Uredo,  64. 
Uredospore,  63,  64. 


Vaccinium,  269. 

Vascular  bundle,  232, 234,  287,  291. 

Vascular  cylinder,  234,  287. 

Vascular  system,  129,  139. 

Vaucheria,  26,  27,  28. 

Vegetative  multiplication,  9. 

Veins,  141,  142. 

Venation,  233. 

Verbascum,  275. 

Verbenacea?,  275. 

Vernation,  143. 

Vernonia,  279. 

Veronica,  275. 

Vicia,  265. 

Violet,  211,  229. 

W 

Wall  cell,  180. 

Walnut,  256. 

Water,  83,  314. 

Water  ferns,  158. 

Water-lily,  223,  261. 

Water-lily  family :  see  Nymphaea- 

ceae. 
Water  moulds,  51. 
Wheat  rust,  63,  64,  65,  66. 
Willow :  see  Salix. 
Wind,  315. 
Wintergreen :  see  Gaultheria. 


348 


INDEX 


Wistaria,  265. 
Witches'-broora,  60. 
Wormwood :  see  Artemisia. 

X 

Xanthium,  279. 

Xerophytes,  319. 

Xylem,  285,  287,  288,  290,  292,  294. 


Yeast,  62. 


Zannichellia,  237. 
Zoospore,  10. 
Zygomorphy,  228, 
Zygospore,  15. 
Zygote,  15. 


THE     END 


(10) 


